Length of Pregnancy

Method of Calculation

Dr. Brown bent back in his swivel chair, tapped together the ends of his fingers, and asked gravely, ‘When did your last menstrual period begin?’

Rustling unsuccessfully in her handbag for the forgotten memorandum, young Mrs. Smith replied, ‘Let’s see. It was the day of the last concert, you know. Let me think, wasn’t it Wednesday, March eighteenth?’

With telegraphic speed, and without the aid of either fingers or calendar—the use of these marks the neophyte—the wise man announced, ‘Your baby is due on Christmas Day.’ Mrs. Smith was impressed! It was barely the first of May, and yet this seer foresaw the birth of her child, seven and a half months distant. How did he do it?

The calculation of the expected date of confinement is very simple, absurdly simple. I hesitate to divulge the formula for fear of revealing a guild secret. The rule is: Add seven days to the first day of the last normal menstrual period. Count back three months. In the case of Mrs. Smith, Dr. Brown added seven to March 18, and then counted back—February, January, December. That made the expected date of confinement December 25. The mystic formula is now exposed. In reality, this formula affords a short cut for counting 280 days from any fixed date. In other words, a woman ordinarily delivers nine months and seven days from the beginning date of her last menstrual period.

It must be stressed that the 280 days is an average figure, which means that a vast number of pregnancies terminate before the 280th day, a vast number after it, and only relatively few on the exact day. At best the calculated or expected date of confinement is an approximate date. This is an important fact for the pregnant couple as well as relatives and other interested persons to remember. It is all too common for panic to become general when ‘B’ days arrives, then passes, and yet there is no sign of labor. Telephones soon begin ringing, and on each occasion the patient is unhappily greeted by the salutation, ‘Haven’t you gone to the hospital yet?’

What Are the Chances of Delivering on Time?

In a recent study of over 17,000 cases of pregnancy carried beyond the twenty-seventh week, 54 per cent delivered before 280 days, 4 per cent on the 280th day, and 42 per cent later. Forty-six per cent had their babies either the week before or the week after the calculated date, and 74 per cent within a two-week period before or after the anticipated day of birth.

On the basis of these data one can calculate the likelihood which the average woman faces when carrying a single infant, not twins, of having her baby, during each week after the twenty-seventh week from the first day of her last menstrual period.

Factors Affecting the Delivery Date

Ordinarily the woman with a consistent, regular menstrual cycle is more likely to have a baby at the 280th day than the woman who menstruates irregularly. Furthermore, a short menstrual interval, such as twenty-five days, is frequently associated with delivery a few days early, a lengthy menstrual cycle with birth beyond the due date. Neither age, race, size, nor the previous number of children seems to influence the length of pregnancy.

A twin conception shortens pregnancy by about three weeks; actually, the average woman who carries twins delivers them on the 258th day instead of the 280th. Triplet and quadruplet pregnancies are usually briefer than this. Mrs. Dionne delivered her famous quintuplets on the 219th day.

A Prolonged Pregnancy

A pregnancy carried more than two weeks beyond the calculated date is considered prolonged, and the resulting infant is described as postmature. Such a delivery occurs approximately once in sixteen pregnancies.

There are some apparently authentic cases in which pregnancy extended to 336 and 337 days, and one in which the duration was 343 days. When pregnancy is excessively protracted there are three possibilities: an error in menstrual dates; ovulation several weeks later than the usual fourteenth day of the cycle, impregnation therefore not taking place until forty or fifty days after the onset of the last menses; or actually several extra weeks of pregnancy beyond the usual forty weeks before labor commences. In most cases the true answer is never known.

Evidence is accumulating that a baby in the uterus gains little weight after the term date of 280 days is reached, so that the birth of a baby of excessive size adds little proof of postmaturity. As a matter of fact, babies may actually lose weight in the uterus after the due date is reached, and it is thought by some authorities that the typical postmature baby is thin, scrawny, and old-looking, with loose, baggy skin. They may also show desquamation, or peeling, of the superficial skin of the palms and soles. After birth such infants gain back the weight they lost and soon appear normally chubby and well padded. In addition, in such cases the surface of the placenta often displays thick deposits of calcium, perhaps evidence of its relative senility.

Calculating the Delivery Date from the Day of Insemination In calculating the expected date of confinement from 425 cases in which a purported single, fruitful coitus led to impregnation, it was found that the average patient delivered 269.9 days after insemination. However, there was wide variation, extending from 231 to 329 days. A second study, involving fifteen cases of artificial insemination, yielded an average duration of pregnancy of 272 days from the day of treatment, with a span of 261 to 288 days. It is obvious that calculating the anticipated delivery date from coital data has little or no advantage over the more standard technique of utilizing the first day of the last menstruation.

Calculating the Delivery Date from the Onset of Fetal Movements

Another method of computing the ‘due date’ is to count eighteen or twenty weeks from the time the patient first feels fetal movements; however, this is even less exact than the calculations from menstrual and coital data.

After sifting all available modern scientific data, we come to the conclusion that the generalization first made decades ago about the duration of pregnancy is relatively correct. If the date is calculated from the onset of the last menses, almost 50 per cent will deliver within the week before or after the expected confinement day, and 75 per cent within two weeks of it.

The Effect of Prolonged Pregnancy on the Fetus

There is no clear-cut evidence one way or the other whether carrying the baby three or four extra weeks jeopardizes its safety. There are a number of studies which demonstrate a slightly increased risk for the fetus, and an equal number which refute such hazard. The final answer awaits a detailed, carefully analyzed, sizable group of authentic postmature cases.

In the meantime, the important thing to do is not to panic and force your doctor into taking unjustified steps when he may rightly feel that the situation calls for no other treatment than the time-tested method of ‘letting nature take its course.’ On the other hand, if the cervix is ‘ripe’ and ready for induction of labor, he may think it wise to take you into the hospital and simply rupture your membranes—the bag of waters—or initiate contractions through the administration of Pitocin or Syntocinon. It is difficult to justify Cesarean section for postmaturity unless there are complicating factors such as the rare occurrence of an excessively large baby which appears too big for safe vaginal delivery.

Determining the Duration of Pregnancy

The physician determines the duration of pregnancy in a given case from the patient’s history and from his findings on physical examination. Usually they agree. From thousands of observations we know that a pregnant uterus of a certain size represents a conception of a certain number of weeks. For measurement, three abdominal points have been selected: the front of the pelvic bone (symphysis), the navel (umbilicus), and the tip of the breastbone (xiphoid). Sometime during the third month the uterus can be felt above the pelvic bone; it is felt midway between the pelvic bone and the navel at the end of the fourth month, at the navel in the fifth, and midway between the navel and the breastbone at the end of the seventh. At the beginning of the ninth month the top of the uterus is two and a half inches below the end of the breastbone. In most women pregnant for the first time the uterus now begins to descend in the abdomen because the child starts to sink into the pelvis. The layman calls this ‘lightening’ or ‘dropping’; medically it is termed ‘engagement of the fetus.’ Frequently in those who have had previous pregnancies the uterus continues to grow upward until the fortieth week, virtually reaching the xiphoid, since the engagement of the fetus may not take place in them until labor has begun. The earlier engagement of the fetus in a first pregnancy is due to the greater pressure which surrounds it, since the uterus and abdominal wall have not been stretched by previous childbirth.

If the menstrual history and the physical examination do not agree as to the duration of pregnancy, the doctor must investigate the cause of the discrepancy. Either there is an error in the menstrual history, or the uterus, because of some abnormality of pregnancy, does not correctly indicate the duration by its size. The uterus may be abnormally enlarged by tumors, multiple pregnancy, excess of fluid (hydram-nios), etc. It may be small because the fetus has died in utero, or the child’s development may have progressed very slowly.