Niddah 38a ~ How Long Does A Pregnancy Last?

If you have ever stayed up all the way to the end of a Pesach Seder, you will have sung the following:

תשעה מי יודע
תשעה אני יודע
תשעה ירחי לידה

Who knows Nine?

I know nine.

Nine are the months of a Pregnancy.

Well I have some news - it ain’t necessarily so. It all begins on today’s page of Talmud:

נדה לח,א

 אמר שמואל אין אשה מתעברת ויולדת אלא למאתים ושבעים ואחד יום או למאתים ושבעים ושנים יום או למאתים ושבעים ושלשה 

Shmuel says that a woman becomes pregnant and gives birth only after 271 days, which is a full nine months, or after 272 days or after 273 days.

As Rashi explains, nine (thirty day) months is 270 days following intercourse. On day 271, the start of the tenth month of pregnancy, the baby is born. Sometimes though, the egg is not fertilized for 24 or even 48 hours, in which case birth happens on day 272 or 273 after intercourse.

Today’s question is, was Shmuel, the famous second century Babylonian physician-rabbi (and head of the yeshiva of Negardea) actually correct?

“
The length of human gestation varies considerably among healthy pregnancies, even when onset of pregnancy is measured by an accurate marker of ovulation. This variability is greater than suggested by the clinical assignment of a single ‘due date’.”

— Jukic A.M. et al. Length of human pregnancy and contributors to its natural variation Human Reproduction 2013. 28 (10): 2848–2855

The length of gestation calculated by the time from the last menstrual period

Generally speaking, studies that examined the length of gestation have estimated gestational age either by the mother’s last menstrual period (LMP) or by ultrasound, both of which are imperfect measures. Based on the first date of the LMP, the end of gestation is 280 days later. But that is not the length of time to which Shmuel referred. He was very precise, and calculated the duration (or received a tradition that was) based on the number of days post conception, Shmuel assumed conception would be on the date of intercourse, or no no more than two days later.

Estimated Due Date (EDD) Calculator. Based on the standard 280 days after the first day of the mother’s last menstrual period.

The actual median length of gestation

In 2013 a study funded by the National Institutes of Health measured things a little differently. It calculated the length of gestation beginning at ovulation or conception in 125 naturally conceived, singleton live births, and determined the exact date of ovulation and implantation by measuring urinary hormone measurements. What they found was very close to Shmuel’s length of gestation. The median time (which is not the average time) from ovulation to birth was 38 weeks and 2 days, or 268 days. That’s a difference of only two days! That’s strong work from Shmuel, who figured it out without the resources of the National Institutes of Health (annual budget, about $32 billion).

Smoothed distribution of length of gestation (accounting for medical interventions that shortened gestation) derived from 125 singleton live births. Solid line: ovulation-based length of gestation. Dashed line: LMP-based length of gestation. From Jukic A.M. et al. Length of human pregnancy and contributors to its natural variation Human Reproduction 2013. 28 (10): 2848–2855.

There is an important caveat, however. The study found that the gestational length varied by 37 days (even after excluding preterm births or pregnancies with medical conditions). This means that the length of human pregnancies can vary naturally by as much as five weeks, and - this is important- that much of this reflects natural variation.

But other factors can influence the length of a pregnancy. For example, older women deliver later on average, with each year of age adding roughly one day to their pregnancy. And women who had themselves been heavier at their own births had longer gestations, with each 100g in the mother's own birth weight corresponding roughly to a one-day longer pregnancy.

The practice of early pious ones

All of which leads us to question the practice of the early pious ones, as they are called by the Talmud. Today’s page of Talmud describes how this group was so convinced by the tradition that gestation was exactly 270 days that they would only have intercourse on a Wednesday, Thursday or Friday. They did this to avoid the risk of the baby being delivered on the Shabbat, and any desecration of it, albeit legally permitted, that might occur to care for the infant or its mother.

Their thinking went like this: 270 days is 38 complete weeks and four days. This meant that delivery would occur four days later in the week than had conception. So if the baby was conceived on a Wednesday, it would be delivered on a Sunday, or a Monday (if fertilization was delayed by 24 hours) or a Tuesday (if fertilization was delayed by 48 hours.) Similarly, a baby conceived on a Thursday could only be born on a Monday, Tuesday or a Wednesday. A baby conceived on a Friday could only be born on a Tuesday, Wednesday or a Thursday. But if the baby was conceived on a Sunday, Monday or Tuesday, they ran the risk of it being born on a Shabbat.

דתניא חסידים הראשונים לא היו משמשין מטותיהן אלא… מרביעי ואילך

As we know, nature does not always cooperate; gestation may on average be about 270 days after conception, but that is an average (or more precisely a median). Some pregnancies are naturally shorter and some longer. Dr Jukic, the lead author of the paper that recalculated the length of human gestation noted that “natural variability may be greater than we have previously thought.” And that is certainly the case for Shmuel and the early pious ones, who calculated the length of gestation with great precision, but alas, not great accuracy.

Yevamot 80a~ The Premature Child is like a Stone

[This was originally posted on Yevamot 42a. Since we have many new readers since then, it's going up again, with only a couple of slight differences.]

“...it is the women who make the judgments and ... insist that the eighth-month babies do not survive, but the others do.
”

— Hippocrates, On the Seventh-Month Child

...בן שמנה הרי הוא כאבן ואסור לטלטלו אבל אמו שוחה עליו ומניקתו מפני הסכנה

A child born in the eighth month of pregnancy is (not going to live and so is) treated like a stone; it is forbidden to move him (on Shabbat). However his mother may bend over him and may nurse him because of the danger (to her if she does not do so...)

This belief - that a fetus of seven months gestation may survive, but one born in the eighth month of gestation cannot do so - is very odd. But it wasn't a uniquely Jewish belief.

Homer's Iliad, written around the 8th century BCE,  records that a seven month fetus could survive. But it is not until Hippocrates (c. 460-370 BCE, or some 500 years before Shmuel), that we find a record of the  belief that a fetus of eight months' gestation cannot survive, while a seventh month fetus (and certainly one of nine month gestation) can.  His Peri Eptamenou (On the Seventh Month Embryo) and Peri Oktamenou (On the Eight-Month Embryo) date from the end of the fifth century BCE, but this belief is viewed with skepticism by Aristotle.

In Egypt, and in some other places where the women are fruitful and are wont to bear and bring forth many children without difficulty, and where the children when born are capable of living even if they be born subject to deformity, in these places the eight-months' children live and are brought up, but in Greece it is only a few of them that survive while most perish. And this being the general experience, when such a child does happen to survive the mother is apt to think that it was not an eight months' child after all, but that she had conceived at an earlier period without being aware of it.

The belief that an eight month fetus cannot survive has a halakhic reification: Maimonides ruled that if a boy was born prematurely in the eighth month of his gestation and the day of his circumcision (8 days after his birth) fell out on shabbat, the circumcision - which otherwise would indeed occur on shabbat, is postponed until Sunday, the ninth day after his birth. 

ומי שנולד בחדש השמיני לעבורו קודם שתגמר ברייתו שהוא כנפל מפני שאינו חי... אין דוחין  השבת אלא נימולין באחד בשבת שהוא   יום תשיעי שלהן     

(הלכות מילה 1:11)

This belief persisted well into the early modern era. Here is a state of the art medical text published in 1636  by John Sadler.  Read what he has to say on the reasons that an eight month fetus cannot survive (and note the name of the publisher at the bottom of the title page-surely somewhat of a rarity then) : 

John Sadler. The Sicke Womans Private Looking Glasse. London 1636. From the Collection of the National Library of Medicine, Bethesda MD

Saturn predominates in the eighth month of pregnancy, and since that planet is "cold and dry"," it destroys the nature of the childe". That, or some odd yearning of the child to be born in the seventh but not the eight month (according to Hippocrates) is the reason that a child born at seven and nine months' gestation may survive, but not one born at after only eight months.

Today, gestational length is of course critical, and, all things being equal, the closer the gestational length is to full term, the greater the likelihood of survival.   We can say with great certainty, that an infant born at 32 weeks or later (that's about eight months) is in fact more likely to survive than one born at 28 weeks (a seven month gestation.) In fact, a seven month fetus has a survival rate of 38-90% (depending on its birthweight), while an eight month fetus has a survival rate of 50-98%. Here is the data, taken from a British study.

Draper Elizabeth S, Manktelow Bradley, Field David J, James David. Prediction of survival for preterm births by weight and gestational age: retrospective population based study  BMJ 1999; 319:1093

More recently, a study from the Technion in Haifa showed that even the last six weeks of pregnancy play a critical role in the development of the fetus. This study found a threefold increase in the infant death rate in those born between  34 and 37 weeks when compared full term babies.  

You can read more on the history of the eight month fetus in a 1988 paper by  Rosemary Reiss and Avner Ash.  From what we have reviewed, the talmudic belief in the unusually low survival rate of an eight month fetus (compared to a seven month one) is one that was widely shared in the ancient world. And one that is not supported by any of the evidence we now have.

Yevamot 42b~ The dangers of pregnancy while nursing

If a mother becomes pregnant while nursing, her milk supply will become turbid and (unless an alternative is found) her nursing child may die (Yevamot 42:)

סתם מעוברת למניקה קיימא דלמא איעברה ומעכר חלבה וקטלה ליה 

So how good a contraceptive is nursing? In a word (well, actually two words) it depends. In the first 3-6 months after birth, and if the baby is fed only on breast milk, some claim that breast feeding is a pretty good contraceptive, and is effective about 98% of time. But if mum skips a feed here or there, or if mum's periods have re-started, all bets are off.  Here's data from an old paper on the topic. Take a close look at the last column- the failure rates per 100 women.

Prolonged Breastfeeding as a Birth Spacing Method Jeroen K. Van Ginneken. Studies in Family Planning, Vol. 5, No. 6 (Jun., 1974), pp. 201-206

Those are high failure rates -as high as one in five - which makes it a pretty unreliable contraceptive. A review of breastfeeding as a contraceptive was published in 2003 in the widely respected Cochrane Reviews; it concluded that "[f]ully breastfeeding women who remain amenorrheic have a very small risk of becoming pregnant in the first 6 months after delivery when relying on lactational sub fertility". However, - and this is really important - it is not possible to know when amenorrhea is likely to end, and so an IUD is suggested as additional contraception wherever possible.

Overall, the talmudic suggestion that conception is possible while a mother is breastfeeding her child is, scientifically speaking, spot on.