When the newborn infant weighs less than 51/2 pounds (2500 grams) and more than 14 ounces (400 grams), it is considered to be a premature infant. If the birthweight is more than 5A pounds, it is designated a term infant; if less than 14 ounces, an abortion. In these definitions no reference is made to the duration of pregnancy. This is because duration of pregnancy is so difficult to determine with exactitude, since menstrual periods are so variable and menstrual data frequently so unreliable. Ordinarily a 5 ¼ -pound baby is four weeks from its expected delivery date and a 14-ounce infant is delivered in about the twentieth week of pregnancy.
Of all the babies bom in the United States during 1959, 7.7 per cent weighed between 14 ounces and 5 ½ pounds. The incidence varies with the socio-economic situation and the race. As stated in the Scottish study the most privileged tenth of the population had a 4-per-cent prematurity rate and the less privileged, 8 per cent. Then, too, since the Negro normally has smaller babies, that race has a higher proportion of infants weighing under 5 ½ pounds at birth. In 1959 the white incidence of prematurity was 6.7 per cent and the Negro, 12.9 per cent. Forty-nine per cent of twins weigh less than 5 ½ pounds at birth; this high rate of prematurity arises from two causes: the usual earlier termination of twin pregnancies over single pregnancies (258 vs. 280 days) and the fact that two infants in the uterus are less well nourished than one.
Other causes of prematurity are toxemia of pregnancy, placenta praevia, premature separation of the placenta, maternal illness with a severe infection—such as pneumonia or syphilis, for example—and there is a large number of cases for which no obvious reason exists.
In these cases an apparently normal patient, most frequently a primipara (a woman who is bearing her first child), either starts in labor or ruptures her membranes when approximately more than twenty weeks pregnant, but less than thirty-six. Occasionally postpartum examination will reveal some congenital abnormality of the uterus or cervix as the cause, such as a bicornuate or double uterus, a uterus with a septum or a tissue partition down its middle, or a cervical tumor such as a low.
When labor starts there is nothing that will really stop it, except occasionally good fortune. If the patient ruptures membranes prematurely, and labor has not yet begun, the physician keeps her flat in bed, perhaps on antibiotics, hoping that the onset of labor will postpone itself. Often it does for several essential weeks.
There is a tendency in some patients for premature labor to recur; in others it is an isolated phenomenon in their reproductive careers. The labors themselves are not unusual, requiring approximately the same length of time as term labors. If premature labor or even late abortion recurs, the patient may be found to suffer from a weakness of the muscle wall of the uterus at a point where it merges with the cervix. In medical terminology, the condition is termed ‘the incompetent os syndrome.’ Diagnosis is made mainly on the basis of the patient’s history and X-ray findings, using a small balloon in the uterus which is inflated with a radiopaque liquid. If the X-ray film shows that the balloon can be withdrawn without being markedly elongated in shape, the picture is considered suspicious. A typical history is abortion after the eighteenth week in which the membranes (bag of waters) rupture before labor commences. In the proper case, an operation can be performed to strengthen and support the weakened ring of muscle by a circular, constricting, broad, non-absorbable suture. The operation is usually performed between the fourteenth and sixteenth weeks of pregnancy. A few operators prefer to do such a ‘circulage’ procedure before such a patient starts another pregnancy. Another type of operation is to cut out a wedge of cervix involving the lower uterine muscle. The wedge is like a slice of cake. The edges of the incision are sewn together, constricting the opening where uterus and cervix join. This latter type of operation is always done in the nonpregnant state. These operations truly work miracles in cases correctly selected. Delivery may be by Cesarean section or vaginally, depending on the type of operation or the variety of suture material. The handling of such a complication requires the services of an experienced obstetrical-gynecological specialist.
The only unfortunate thing about a premature labor, whether its onset be spontaneous or the result of necessary termination of pregnancy prematurely by Cesarean section, is the poor fetal salvage in the low-weight categories.
Of 160 white babies weighing less than 2 pounds, 3 ounces (1000 grams) born alive during 1959 in eleven United States hospitals which combine their statistics under the title, ‘Obstetrical Statistical Cooperative,’ only 1 survived. Of 200 white babies weighing between 2 pounds, 3 ounces and 3 pounds, 4 ounces (1500 grams), 45 per cent lived.
When the birthweight was between 3 pounds, 4 ounces, and 4 pounds, 6 ounces (2000 grams), 84.5 per cent made the grade, and between 4 pounds, 6 ounces and 5 pounds, 8 ounces (2500 grams), 96.3 per cent. Only 0.4 per cent of 29,770 white babies born alive weighing over 2500 grams or 51/2 pounds died.
Non-white babies in the lower weight categories have consistently better survival rates than white babies in the same weight groups. Newborn survival depends significantly upon two factors, birthweight and gestational age (by this is meant the length of the pregnancy). Non-white babies weigh about three-quarters of a pound less at term, at 40 weeks, than white babies, and are proportionately smaller throughout pregnancy. White babies weighing between 2 pounds, 3 ounces and 3 pounds, 4 ounces have a gestational age of 28 to 31 weeks, while non-white babies of the same weight have a gestational age of 30 to 33 weeks. The more advanced the gestational age, the more mature are the infant’s vital organs—lungs, kidneys, liver. This explains why only 45 per cent of white babies weighing between 2 pounds, 3 ounces and 3 pounds, 4 ounces survived in contrast to 62 per cent of non-white infants.