Childbirth and Twins

How Many at a Birth?

We read in the works of Ambroise Pare, premier surgeon-physician of sixteenth-century France, that Lady Margaret, Countess of Hagenau, was brought to bed of 365 children at one and the same time in the year 1313. Milady’s super-fecundity came about in a miraculous way, through God’s displeasure. It happened that, sometime the year before, the countess was walking through the gate of her walled palace when her skirt was plucked by a kneeling beggar woman who said, ‘My good lady, give me alms.’

The countess, haughty and indignant, replied, ‘Why should I give you alms?’

The beggar answered, ‘Because of all the children I have begot.’

The countess looked down upon her in disdain. ‘Fie upon you, you’ve had the pleasure of begetting them.’

God in heaven overheard, we are told, and he was wroth. The next year the countess was lain in with 365 children, one for each day in the year. There were 182 females, all baptized Elizabeth by the good Bishop of Utrecht, and 182 boys, all baptized John. There was one ‘scrat’ (hermaphrodite), who pathetically remained unnamed and unbap-tized. In reporting the same case a hundred years later, Mauriceau said it was either ‘a miracle or a fable’; we are inclined to the belief that it was a little of both.

Other fabulous cases are recorded, such as ‘Dorothy, an Italian who had twenty at two births, at the first nine and at the second eleven, and she was so big . . . her belly . . . rested upon her knees.’

When we descend to the number seven it becomes impossible to know whether we are dealing with fact or fable. In the German town of Hamelin, which the Pied Piper made famous in 1248, we find the following tablet attached to the front of a house on Emmenstrasse: ‘Here on this spot dwelt . . . Thiele Romer, and his helpmate Anna Byers. It came about that in the year 1600 … at three o’clock in the morning on the ninth day of January, she was delivered of two small boys and five small girls. … All peacefully died by twelve o’clock on the twentieth of January.’ That is all we know about the possible, perhaps even probable, Hamelin septuplets.

We come now to undisputed facts, leaving the realm of the fabulous and the pseudo-fabulous. There are at least five authenticated cases of sextuplets. In none did any set or even any member survive. A report of a case from South Africa in 1909 published the pictures of the six infants.

Quintuplets are reported every few years, but so infrequently that it is impossible to state the mathematical frequency. The first set of quintuplets ever to survive in history were the Dionne sisters of Canada—not even a single quintuplet had ever survived before.

Frequency of Various Multiple Births

Hellin in 1895 stated that twins occurred once in 89 births, triplets once in 892 (7921), and quadruplets once in 893 (704,969). Actually, Hellin’s so-called law is wrong on two counts. A sufficiently close mathematical relationship does not exist between the different orders of multiple births so that squaring and cubing the twinning frequency will accurately give the frequency of triplets or quadruplets. Hellin also failed to recognize that the incidence of multiple births varies markedly in different population samples.

In an analysis of the 80 million births which occurred in the United States between 1928 and 1955 we found that quadruplets occurred once in 490,000 pregnancies, triplets once in 9300, and twins once in 90.

What Chance Have You for Twins?

I cannot answer without knowing your race, age, parity (number of children you have conceived), and whether you or your husband have a family tree studded with two apples for each blossom.

What about your race? In vital statistics throughout the world, marked differences are found in the incidence of multiple births among Negro, white, and yellow populations. In this country twins occur once in 73.2 Negro pregnancies, once in 93.2 white births; triplets once in 5800 and 10,200 births respectively. In Japan twins occur once in 154.4 pregnancies, triplets once in 17,200. You have the best chance for twins if you are a Negro, the poorest chance if you are Oriental, and an intermediate chance if you are white.

What about your age? White women under twenty have a very low twinning incidence—1 in 167 births. After twenty it steadily rises, the peak frequency being recorded for the age group thirty-five to forty, when its occurrence is 1 in 59. The frequency then declines, so that women over forty-five have the same poor chance as girls under twenty.

What about your parity? Parity, the number of children already begot, exerts an influence independent of age, but usually the two go hand in hand, for younger women ordinarily have fewer children and older women more children. If you are a white woman of thirty-five to forty in a first pregnancy, you have 1 chance in 74 of bearing twins, but if you are thirty-five to forty having your seventh pregnancy, your chance is increased to 1 to 45.

What about your heredity? A tendency to multiple births is inherited in some families, and it affects both the men and women of these stocks. In other words, not only the women bring this gift as part of their dowries, but the men as well. Such families are distinguished by the fact that their trees have several instances of twinning on almost every branch, not just an occasional sporadic pair, as almost all of us can find if we scan family horizons diligently. The tendency to twinning does not skip a generation; it is expressed in succeeding generations.

Types of Twins There are two types of twins. One variety originates from a single fertilized egg which divides into two very early in its development. In these circumstances, one egg is fertilized by one sperm, and the germ plasm of the two offspring is therefore identical. Consequently they must be of the same sex and exactly alike in skin, hair, and eye color. They also bear a striking resemblance to each other in body build and facial features and possess exactly the same blood factors. Such twins are termed identical, one-egg, or monozygotic.

The other type of twinning results from the fertilization of two different eggs by two separate spermatozoa. The eggs may come from the same ovary or from opposite ovaries. Twins of this variety, known as fraternal, two-egg, or dizygotic, are simply a Litter of two and bear no greater resemblance to each other than brothers or sisters at exactly the same age. They may be of the same or opposite sex and may or may not have the same blood types.

Can You Tell One-Egg from Two-Egg Twins at Birth

If they are boy and girl, they necessarily must be from two eggs, non-identical. If they are of the same sex, your doctor can get either a definite answer or a strong hunch from careful inspection of the afterbirth (placenta). If the two children have a single placenta, and examination of the membranes at the point where the two sacs surrounding the fetuses come in contact to form a partition wall demonstrates only two thin membranes, one can be certain that the twins are identical, from one egg. On the other hand, if each twin has its separate placenta, or examination of the partition wall in the single placenta demonstrates three or four membranes instead of just two, there are nine chances in ten that the twins, though of the same sex, are not identical but fraternal and from two eggs. An absolute decision in such cases must await further tests.

One of these tests can be carried out at birth, a study of the two bloods. The two bloods must be exactly alike in all respects: Rh, major groups (A, B, AB, O), and minor groups such as M, N, Kell, etc. If the bloods differ even in the slightest, one can be sure the twins are not identical. If the bloods are exactly alike, ordinarily final decision must await physical-resemblance studies when the twins are a few years old. To be indentical the two must have the same coloring and same body builds, and must be so alike that they are difficult to distinguish. However, a final and absolute test to establish the identity or non-identity of a pair of like-sexed twins is the use of reciprocal skin grafts. If the twins are from one egg, a small piece of skin grafted from twin A to twin B will ‘take’ as perfectly as though the graft were taken from A and grafted to A. If they are from two eggs, the graft will wither and not ‘take.’ This viability of grafts from one identical twin to the other member of the pair has been utilized in transplanting one kidney from a healthy twin to his co-twin dying from kidney failure. Five of eight such operations have succeeded.

Frequency of the Two Types

In the United States, among whites, 34 per cent of all twins are identical, while 66 per cent are fraternal; among Negroes 29 per cent are identical and 71 per cent fraternal. In direct contrast, the yellow populations studied have all shown the majority of their twins identical.

From this it becomes obvious that those racial groups with a high frequency of twinning have a large proportion of fraternal, two-egg twins; while those groups with a low incidence of twinning have a relatively small proportion which are two-egg. Further study has shown that one-egg identical twinning occurs with the same fixed frequency in all people, irrespective of race, age, parity, and family history. The variable, the factor which affects both the frequency of twinning and the proportion between the two types, is the occurrence of two-egg, fraternal twinning, which is highly sensitive to race, age, parity, and family history.

Triplets, Quadruplets, and Quintuplets

The same factors influencing the occurrence of twins similarly affect the production of triplets, quadruplets, and quintuplets.

One or several eggs may be involved in the creation of multiple pregnancies of the greater magnitudes. Quintuplets, for example, may come from one, two, three, four, or five eggs. On the basis of physical-resemblance studies by a group of Canadian scientists, the Dionnes are known to have arisen from one egg. The proportion of the various egg combinations has been studied for triplets. In every ten cases: in six, two eggs are involved (a pair of identical twins plus a singleton); in three, all come from one egg (identical triplets); and in one, three eggs are fertilized (fraternal triplets).

Siamese Twins

Siamese, or conjoined, twins are always one-egg twins whose misfortune was that the embryonic area failed to split completely, so that the ovum only partially divided. The undivided portion is that part of the body that the conjoined twins have in common; the divided portions are those parts of the body that are separate in the two individuals.

Conjoined twins are predominantly female, and the commonest variety are twins joined back to back in the regions of the sacrum, called pygopagus. They ordinarily have many vital structures in common, including the rectum and, in females, the vagina. So far, live pygopagus twins have defied surgical separation with the survival of either, although such operations have been successful when less vital structures are involved.

There have been many famous pairs of Siamese twins; their lives have elicited much curiosity, which has resulted in complete biographies of several. The stories of two such cases follow.

Eng and Chang were born in Siam in 1811, the fourth pregnancy of a half-Siamese, half-Chinese mother by a Chinese father. The boys were united by a pliable bridge of tissue four inches in diameter, reaching from the lower end of the breastbone to the navel. As the connecting link allowed much freedom of movement, they were both able to face forward if they tired of looking at each other. They were not identical in health, strength, or temperament. Eng, the right twin, was the stronger and healthier of the two and had the better disposition. When eighteen, they came to America and soon fell under the golden direction of the great showman P. T. Barnum, who made them rich and famous.

After amassing financial competence, the twins retired to North Carolina, where they raised tobacco and children. Eng’s wife, Sally, had twelve children, and Chang’s wife, Adelaide, ten. The wives were sisters, daughters of a Virginia clergyman, and delivered their first children within three or four days of each other. Things went well until the wives, resenting the unnatural intimacy forced upon them, began to quarrel, compelling the two families to take up separate abodes a mile and a half apart. Eng and Chang arranged to spend three days in one home and then three days in the other. The one whose home they occupied was complete master over their joint destiny for the three-day period.

The Civil War destroyed their property, and when nearly fifty-five the Siamese twins had to take to the road again. In 1872, when sixty-one years old, Chang had a paralytic stroke which so incapacitated him that his healthy brother had to half-drag the paralyzed victim about for the two years they survived. On the day of death Eng awoke and tried to arouse Chang; not succeeding, he called for one of his sons, who also failed to awaken the twin. Terrified, the son called out, ‘Uncle Chang is dead.’

Eng sighed and exclaimed, ‘Then I am going also!’ Soon he complained of a feeling of suffocation and desired to be propped up. He became weaker and weaker, and a little more than two hours after he discovered his brother’s death he died.

The dead twins were transported by special train to the University of Pennsylvania, in Philadelphia, where an autopsy was performed before the most distinguished groups of physicians ever till then gathered in America. Chang, the autopsy report shows, died of pneumonia. Eng was frightened to death, it claims. The band of union, it was found, could easily have been severed even in 1874.

Rosa and Josepha, Bohemian twins, born in 1878, were united at the base of the spine and had a common rectum and vagina, though separate uteri. As far as I know, they are the only female pygopagi to bear a child. It is stated that the two had one husband. At the age of thirty-two Rosa gave birth to a normal son after a very short labor. Josepha did not experience the pains of labor, and both women were equally able to nurse the infant. They died a few hours apart in Chicago during the influenza epidemic of 1918.

Superfetation—Conception during Pregnancy

Superfetation means’ conception during pregnancy. One of the medical worthies of yesteryear phrased it very succinctly —’a reiterated conception.’ In some cases the interval between conceptions is thought to be several months, so that a term child is born with an immature fetus as co-twin. This presupposes ovulation during pregnancy, the passage of spermatozoa through the pregnant uterus into the tube, fertilization of the second egg, and its downward passage into the uterus, where it must implant alongside its co-twin, which has had a several-week or several-month start This is theoretically possible until the fourteenth week of pregnancy, when the uterine cavity is completely sealed off by the enlarging fetal sac. After such an obliteration of the uterine cavity, it is theoretically impossible for spermatozoa to ascend.

The evidence in favor of superfetation is of two kinds. The first is the simultaneous birth of two fetuses with great disparity in size and birthweight. Cases have been reported with both twins born alive, one weighing 3110 grams (6 pounds, 14 ounces) and the other 420 grams (14 ½ ounces). This extraordinary difference could be due to superfetation or to inequalities of nutrition during intrauterine existence, so that one twin was luxuriously nurtured, while the other, due to an abnormal insertion of the cord or some other biologic phenomenon, was grossly undernourished.

The second evidence in favor of superfetation is the birth of living twins many weeks or even months apart. A recent case reported from Australia tells of the birth of a twin of 3 pounds, 103A ounces, and the delivery of its co-twin, 5 pounds, 1434 ounces, 56 days thereafter. Both twins survived, the first having reached the weight of 6 pounds when the second was bom. It is almost an infallible rule that when twins are born more than seventy-two hours apart, the mother has a double uterus and a gestation in each. One uterus goes into labor, and the woman bears a premature infant; a few months later the other uterus empties itself of the term-sized co-twin. The birth, then, of living twins many weeks apart is no real evidence of superfetation.

Despite the fact that superfetation has apparently been proved for the rat, its proof for the human remains uncertain. Until such proof is adduced, we feel that superfetation is simply a theoretical possibility.


Superfecundation is the fertilization of two ova within a short period of time by spermatozoa from separate copulations. It is only distinguishable from usual two-egg twinning if the female has coitus with two males with diverse physical characters, each passing his respective traits to the particular twin he has fathered, or when the study of the blood groups of the mother and the twins demonstrates that one man could not have fathered both. Superfecundation is commonly recognized in animals and has also been observed in the human. One of the early authentic cases was reported by John Archer of Maryland in 1810. In this case, the white mother gave birth to a white child with a mulatto as co-twin. On questioning, it was determined that she had had coitus with her white husband and with a Negro within a few hours.

Fertility of Twins

There is an old canard that twins are less fertile than singletons. This probably has its source from the fertility difficulty that the heifer co-twin of a bull calf is likely to encounter. In 95 per cent of such cases, the female litter mate, termed a freemartin, has no uterus and ovaries, and has an undeveloped vagina, for in cattle twins of opposite sex the testis develops before the ovary and passes its hormone into the circulation of the female. This is possible in cattle because of a unique type of placenta; in them even two-egg twins have a very extensive connection of the blood vessels of one placenta with the blood vessels of the other. The transfusion with hormones from her brother’s testis inhibits in the heifer the development of ovaries and of the whole female generative tract. There is no such phenomenon in the human, whether the twin pair are of opposite or same sex. As early as 1839 the famous Scottish obstetrician, Mathews Duncan, submitted the fertility of human twins to statistical analysis and found it no different from that of those singly born,

The Causes of Twinning

A consideration of the causes of twinning brings out more clearly than anything else the inherent differences in the biology of one- and two-egg twinning. The one-egg variety is simply a biologic phenomenon that occasionally occurs in all mammals, including subhuman primates. There is suggestive evidence from comparative biology that deleterious influences exerted upon the ovum just after fertilization increase twinning and other forms of pathologic tissue doubling. It is likely that in man the most frequent deleterious influence is the temporary reduction of oxygen through one of several theoretical mechanisms.

Two-egg twinning is a very different phenomenon, resulting from, of course, a double ovulation. It is simple to explain the hereditary transmission of the tendency to produce two-egg twins through the female. In such cases we must either postulate an abnormal tendency to double ovulations in the stock, or an unusual ability to have fertilized and to implant and develop every egg ovulated. To explain the hereditary tendency through the male is not so simple. One must either hypothesize spermatozoa of great potency which are so utterly normal that all ova are fertilized and develop, or that the spermatozoa of the particular family have been endowed with an uncommon ability to fertilize a polar body —a by-product extruded from the normal egg just before it is ripe for fertilization. The polar body contains the twenty-three chromosomes cast off by the egg to halve its number of chromosomes from forty-six to twenty-three. Both of these hypotheses are purely conjectural, particularly the latter hypothesis, since we lack proof that a polar body is ever fertilized.


Twins are diagnosed before delivery in about seven out of ten cases, those undiagnosed being as great a shock to the doctor as they are to the patient. The larger the infants and the nearer to term the pregnancy, the less likely twins are to be missed. If either twin weighs five and one-half pounds or more, there are four chances in five that the diagnosis will be made during pregnancy.

The following observations by the physician create in his mind suspicion of twin pregnancy. 1. A uterus, appreciably larger than is usual for the duration of the pregnancy, which continues to grow at an accelerated rate. Twins must be differentiated from hydramnios (excessive amniotic fluid), fibroids (uterine muscle tumors), hydatidiform mole (replacement of the pregnancy by abnormal grapelike structures), and a very large single child. The doctor’s suspicions are likely to be aroused by this during the sixth month. 2. Excessively rapid weight gain in the absence of edema (watery swelling of the tissues), hydramnios, or quickly increasing obesity. 3. A small presenting part in the pelvis on vaginal examination, with the cervix markedly shortened and two centimeters or more dilated, four weeks or more before term, without evidence of labor. 4. The complaint by the patient of excessively pronounced fetal movements, especially if she has had children and compares fetal movements in the current pregnancy with others.

The doctor can be sure that twins are present if the following conditions prevail: 1. He can feel two heads or two breeches by abdominal examination. 2. He can distinguish a fetal heartbeat over two separate areas of the abdomen. The rates should differ by ten beats or more per minute. The observation is best made by two persons, each counting one of the heartbeats. If the two heart rates are nearly alike, one fetus may be stimulated by deep palpation of the abdominal wall; the stimulated fetus will react by an increased cardiac rate, which then becomes distinguishable from that of the co-twin. Two fetal hearts may be picked up any time after the fifth month, but usually are not detected until the seventh or eighth month. 3. He can discern parts of two fetal skeletons upon X-ray of the mother. These may appear by the eighteenth week or earlier, but the presence of a second skeleton cannot be completely ruled out until about the twenty-fifth week. 4. By special technique a fetal electro-cardiogram can be taken through the mother’s abdomen. As early as the twelfth week it may reveal two fetal heart tracings. When two fetal complexes are recorded twins are a certainty; if only one is found it does not rule out twins.

Length of Pregnancy

In our study of twinning we found that the average length of pregnancy was 257.8 days—that is, labor occurred 22.2 days before the calculated date of confinement

Discomforts of Pregnancy

I truly sympathize with the woman pregnant with twins; the minor discomforts are doubled just as much as is the number of fetuses. Breathlessness, varicose veins, hemorrhoids, insomnia, and swelling of the legs and even of the vaginal lips are all too common.

Complications of Pregnancy

Certain complications are particularly frequent in multiple pregnancy. Prominent among them is hydramnios, which makes the already distended abdomen even more distended. The excessive water usually involves only one of the two fetal sacs and is seen in the case of one-egg twins more commonly than with two-egg twins. Treatment consists of bed-rest and sedatives when necessary to relieve discomfort; in very extreme cases the uterus is tapped with a needle and some of the fluid withdrawn. Pre-eclampsia and eclampsia, the two specific high-blood-pressure conditions associated with pregnancy, show an increased frequency. Since two fetuses are present, and either one excessively large placenta or two placentas, the unknown factor that causes such blood-pressure complications, perhaps produced by the placenta, probably also occurs in excessive quantities. Anemia occurs more frequently when a woman carries twins, since there are two fetuses which parasitically deplete maternal iron stores. Weight gain is likely to be excessive. Part of the excess is due to the additional fetus, placenta, and amniotic fluid (ten pounds), and part to an excessive retention of tissue fluids. This can be observed in the greater tendency of the patient with twins to develop dependent edema (swelling of the lower limbs).

Conduct of a Multiple Pregnancy

The doctor’s special care of a woman pregnant with twins re- volves around two main considerations: her greater likelihood of premature labor, and her increased liability to toxemia.

When twins are diagnosed, the pregnant woman is advised to avoid physical strain by stopping work at twenty-four weeks instead of the usual thirty-four weeks. She also must give up travel, not only because it may possibly make premature labor more likely in a twin gestation, but because, since labor may come on at any time without warning, absence from her home city may be hazardous as well as inconvenient. The patient should take long afternoon rest periods. Coitus should be eliminated during the last three months because of the possibility of a prematurely dilated cervix or the early onset of labor. The patient is placed on a salt-poor diet. The frequency of prenatal visits is also increased to facilitate detection of incipient pre-eclampsia. Since anemia is so common in multiple pregnancy, iron is usually prescribed. When anemia is present and refractory to iron, transfusion may be necessary.

If on vaginal or rectal examination the doctor finds that the cervix is materially shortened and partially dilated, and pregnancy is not yet far enough advanced to be sure of two good-sized babies, he may recommend complete bed-rest in the hope that it will forestall labor’s onset


Most twin labors are satisfactory. Because of the smaller size of the fetuses and the fact that the cervix frequently is partially dilated before labor begins, the ordinary plural labor is shorter than the ordinary single labor. Yet there is a greater proportion of unsatisfactory, dilatory labors among plural births than among single births, due in the main to an increased incidence of uterine inertia and poor, inadequate contractions. The overdistention to which the uterus is subjected is probably the cause. False labor is more frequent in multiple than in single pregnancy.

The membranes are more likely to rupture before the onset of labor in multiple pregnancy (29 per cent) than in single pregnancy (12 per cent).

In regard to fetal position in twins, one finds all the possible combinations and permutations for two fetuses, either of which may assume any of three positions—head, breech, or transverse. One presenting head first, the other breech, is the most frequent combination; although both presenting as cephalic is almost as common.

Conduct of Labor

Since labor complications are somewhat increased in multiple pregnancy, whenever possible such deliveries should be conducted in a hospital. Furthermore, while in labor, each patient with a multiple gestation, when possible, should have a matched bottle of blood held in readiness. Almost certainly it will not be used, but having it available gives the doctor a relaxed feeling, just in case.

If rupture of the membranes occurs before the onset of labor, the patient is likely to be hospitalized and may be put on prophylactic antibiotics. Because of the frequent small size of the fetuses, a twin labor must often be conducted with a minimum of analgesia, since the respiratory center of the small fetus is more easily depressed by such drugs.

Conduct of Delivery

The best fetal and maternal results are obtained in multiple births when the least operative interference is employed.

If the first child presents as a vertex or a breech, delivery is carried out as if it weise a single child. An episiotomy is done routinely unless the vagina is relaxed and the infant small. After the first child is delivered, the cord is clamped and cut. The placental end of the cord is carefully checked for bleeding, since in single-egg twins hemorrhage from this source might be fatal to the unborn child. Today, five to ten minutes is allowed to elapse after the birth of a first twin before commencing the delivery of the second. The obstetrician inserts his hand into the vagina and determines the presenting part of the second twin and, by combined vaginal-abdominal examination, its exact presentation. If the head or breech is over the inlet, the second sac is ruptured and the presenting part guided into the pelvis by the vaginal hand, as the abdominal hand makes downward pressure on the uterus. If the second fetus lies in the oblique or transverse, frequently it can be maneuvered into a longitudinal presentation, and then the membranes can be ruptured. After the presenting part of the second child engages in its longitudinal axis, pressure on the uterus usually causes the head or breech to descend rapidly to the pelvic floor, as the delivery of the first fetus just minutes before has dilated the soft parts so that they offer no resistance. After the head or breech of the second child descends, it can be delivered either spontaneously or by simple operative means. If the second fetus cannot be manipulated into its longitudinal axis, an internal podalic version and extraction must be performed.


Some physicians prefer conduction anesthesia, usually saddle-block, for twin deliveries; while the majority, with which I align myself, claim better results with general anesthesia, either gas-oxygen-ether or cyclopropane-ether.

Post-Delivery Care

Care after delivery of the mother who delivers twins does not differ essentially from the care of the woman who gives birth to a singleton, except that during the first few hours she is watched with special vigilance to detect and treat the tendency of the uterus to relax and bleed excessively. Early ambulation is allowed. Lactation is normal, but, unless the amount of milk is copious, nursing is impractical; otherwise both children will require supplementary bottles, which becomes quite a chore.

The Babies

One twin pregnancy in twenty terminates so prematurely that the fetuses each weigh between 14 ounces and 2 pounds, 3 ounces. Only one single fetus in two hundred has a birth- weight in this range. In nine twin pregnancies out of twenty the larger of the twin babies weighs between 2 pounds, ounces, and 5 pounds, 8 ounces; in single pregnancies the chance of such an occurrence is one in twenty. In one-half of all twin pregnancies at least one child has a birthweight of 5.5 pounds or more. In 93.7 per cent of single pregnancies the newborn weighs at least 5.5 pounds.

The average birthweight of single babies is 71/2 pounds; the average birthweight of twins, 5 pounds, 5 ounces. The disparity in size is due to two factors: the earlier termination of multiple gestations, and the relatively unfavorable nutritional environment twins suffer while in the uterus. Twins grow more slowly in utero. When a single baby and a twin baby are carried the full nine months, there is an average difference in birthweight of VA pounds. The difference in length is less marked, being about three-quarters of an inch. Not all twins are small at birth. In our series of 1000 twin infants, 3 per cent weighed 8 pounds, in contrast to approximately 20 per cent of total births; 0.4 per cent weighed over 9 pounds, in contrast to 6 per cent of singletons. The largest twin of our series was 9 pounds, 2 ounces; the heaviest pair totaled 17 pounds. This hardly competes with a pair reported by Holzapfel in 1935, who together weighed 20 pounds, 4 ounces.

Difference in Birthweight between Twins

There may be a tremendous difference in the birthweight of a pair of twins, two or three pounds not being extraordinary. The difference is likely to be greater in identical twins than in fraternal twins. In our study the average pair difference in identical twins was 14 ounces, while in fraternal twins it was 6 ounces. There is no rule as to which twin, the heavier or lighter, will be firstborn. When a pair of twins are of opposite sex, the male averages 3 ounces heavier.

Sex Ratio

Of 126,328 pairs of twins born in the United States, 42,923 were both male, 42,557 male and female, and 40,848 both female. The total of 128,403 male twins and 124,253 female twins produces a sex ratio of 101.6 males per 100 females, instead of the usual in singletons, which is between 105 and 106 males per 100 females.

Results to the Babies

Twins face a four times greater mortality risk than do single infants. The main factor responsible is the relatively large proportion of twins weighing less than four and one-half pounds.

When both twins are born alive there is no consistent difference in survival between twin number one and twin number two unless the second twin weighs less than 41/2 pounds, when its chances for survival are considerably less than that of its co-twin born first.

Since ordinarily at the same weight twins have a greater gestational age than single fetuses (are older in weeks or months spent in the uterus), they do better. In other words, a twin born alive weighing three pounds has a greater chance for survival than a single baby weighing three pounds.

Congenital malformations are slightly more common among twins. Two-egg or fraternal twins never show the same malformation, except through the thin arm of coincidence. On the other hand, it is rare for an identical twin to suffer from a defect not shared by its fellow. Such identity of abnormalities may appear on the same side of the body of each twin or, through the biologic mechanism of mirror-imaging, common in one-egg twins, may occur on the opposite side. This same process of mirror-imaging is responsible for the high frequency of opposite-handedness found in adult pairs of identical twins. In between 30 per cent and 45 per cent of identical twin pairs, one twin is right-handed, the other left-handed.