Please attribute to a British midwife, whose name I no longer recall. She was writing her PhD dissertation on this topic at the time of the lecture.
It was Aristotle who first said that pregnancy was 10 lunar months, which was taken to mean 280 days. From Aristotle’s observation, Franz Karl Naegele created a rule (based after Hermann Boerhaave’s algorithms), a rule of thumb which enabled people to calculate the length of pregnancy – because the Gregorian calendar, which was introduced at the time this debate was going on, was uneven; we can’t divide the lunar year into the solar year, they don’t divide mathematically
And so we’ve had this fascinating argument and problem through the ages about how you do a calendar – because months are important to us because of the moon, and seasons are important to us because of the sun. We can’t divide one into the other, so various societies have had what they call inter-calendar days, where they have something like a month and they insert a few extra days – which is quite confusing. The way we do it at the moment honors the seasons /the cycle of the sun around the year – and then there are months that roughly correlate to the moon, but don’t exactly correlate to the moon.
The rule basically says that you add 7 days and 9 months to the first day of a woman’s last menstrual period, and that’s how you calculate her due date.
However, despite the fact that we’ve been using this for centuries, this rule is completely unsubstantiated by any large-scale research, and was actually based on observation of 100 women, who lived over 100 years ago.
And so, every single woman in the Western world today, and in many other parts of the world as well, are having their length of pregnancy determined by a rule, invented by a couple of guys, who looked at 100 women and said “oh yea, this is roughly what it looks like.”
The idea that 20% of women go “overdue” and their pregnancies last too long is confounding.
As a midwife I can understand if a few women have situations which are pathological in a given context. I can understand that 1 in 300 women will have this particular problem which we need to look out for, but I find it very hard to believe that one in 5 women have bodies that don’t know how long to grow a pregnancy. That’s pretty hard to wrap my head around.
Is it rational that we can’t trust women’s bodies to decide when to go into labor because we would rather trust one European white guy’s observation of 100 women a few hundred years ago? This is pretty much what’s going on.
Most women wouldn’t regard the due date as that important if not for the implication of importance which the medical world imposes on the due date. In our culture being “overdue” is not a desirable quality. If a library book is overdue, you’re in trouble and you get fined. “Overdue” is not a good thing, it’s like “overcooked” – “oh I left the cake in the oven too long, it’s all black & not edible.” Overdue has connotations which are not generally happy in our society.
This midwife asked herself about the importance of dating the pregnancy at all. She concluded for herself that many of the things that we do to a woman in pregnancy are dependent on how pregnant the woman is. Screening tests, rhogam, induction, midwife/doctor to know when to be ready (plan), etc.
But as we clearly need a different standard than 280 days applied to everyone, she offers 3 arguments for due date alternatives:
Case for a Higher Average
Suggests that Naegele was referring to the LAST day of the menstrual period rather than the first day. He actually did not specify. He wrote: “count from the menstrual period.” He did not say the first day. If it was the last day, for example, if a woman has a 3 day cycle we’re looking at 283 days of pregnancy rather than 280 days. Then, remember Aristotle said that the pregnancy was 10 lunar cycles, which is commonly thought to mean 280 days. However, that’s how long it takes for the moon to go around the earth. But the earth moves as well. So it actually takes roughly 29 ½ days for a Sidereal lunar cycle, which takes into account the earth’s movement. If we use the Sidereal lunar cycle on Aristotle’s observation, we would have the average length of pregnancy being 295 days.
Case for Individualization
Lots of midwives talk to women about their menstrual history, their form of contraception; if she has a long or a short cycle they may adjust the due date accordingly. There’s been a series of research studies that have shown that a number of physical factors influence the length of pregnancy – women from different ethnic backgrounds, women who are taller tend to have longer pregnancies than women who are shorter, and age can be a factor as well.
Case for a Wider Window
We should perhaps think of having a “week” of expected birth or a “month” of expected birth. Some midwives say “you’re going to have your baby sometime between the middle of June and the middle of July.” The midwife may have something more specific in her notes or in her head, but they give the woman a range of dates.
This brings us to a normal human gestation period that may be anywhere from 280-295 days, with an EDD widely varying from a rigid 280 day rule. Studies show that Naegele’s Rule should be used as a guideline and not a definite date. Learn more at Supported Birth.