It’s pretty much the first question from everyone who finds out that you’re pregnant. I think on a rational level, the vast majority of people know that you can’t really predict when babies will be born, but I suspect the majority still believe that the Estimated Due Date (EDD) is scientific and accurate at least some of the time. The truth is though, it’s an arbitrary date determined by outdated pseudo science, a best guess, based on the probable misunderstandings of ancient theories.

The theory goes that pregnancy lasts for 40 weeks (9 months + 1 week) from the first day of a woman’s last menstrual period (LMP). Where did this idea come from? It’s called Naagele’s Rule, named after German obstetrician Franz Karl Naegele (1778–1851), who devised the formula. I don’t believe that Naegele plucked this idea out of thin air, it is likely that he read Aristotle’s theory that pregnancy lasts for about 10 lunar months, and Naegele assumed that a lunar month was 28 days.

Aristotle, however, was what I call a “well rounded wise man”. He was a philosopher, mathematician, scientist and sociologist. With his education in physics, it is extremely likely that he knew that a lunar month is not in fact 28 days, but nearly 29.5 days, making 10 lunar months 295 days, NOT 280, over 42 weeks, NOT 40. What a difference to pregnancy length that makes. What Aristotle actually wrote was:

pregnancy may be of 7 months’ duration or of 8 months or of 9 and still
more commonly of 10 (lunar) months, whilst some women go even into the
eleventh month. 

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1341914/pdf/bmjcred00258-0017.pdf

Naegele’s Rule also assumes that all women have exactly 28 day cycles and that they ovulate on cycle day 14. Obviously, this is not true. There is great variation in both cycle length and ovulation timing. There is a suggested alternative formula, Parikh’s Formula, which is a date calculated by adding 9 months to a woman’s LMP, then adding the length of her average cycle, then subtracting 21 days. For example, a woman with 35 day cycles whose last period began on 1st January would do the following calculation:

1st January + 9 months = 1st October

1st October + 35 days = 4th November

4th November – 21 days = 14th October

This EDD is approximately one week later than EDD calculated by Naegele’s Rule.

A small study in 1990 by Mittendorf et al. found that the parity of the woman was a factor in the length of her pregnancy. First time mums had an average pregnancy of 41 weeks and 1 day (41+1), second and subsequent babies arrived at an average of 40+3. Some studies have also found racial variations in pregnancy length.

Normal human gestation is usually defined as 37-42 weeks, by the WHO, maternity professionals and academics, and yet the myth of the EDD persists. Midwives and obstetricians do nothing to correct this misconception, they often seem to fixate on the EDD, first calculated by LMP and then by ultrasound, the infamous “dating scan” that most pregnant women have at about 12 weeks in the UK. (My views on the reliability of ultrasound are explored in my previous post “Guess the Weight”). The dating scan has a 6 day margin for error, officially, and yet it is relied upon so strongly that a woman’s knowledge about her own body is usually dismissed in favour of what the machine says. For instance, were we using NHS maternity services for this pregnancy, the sonographer that did our early pregnancy scan (for reassurance) would have us believe that this baby was conceived 8 days prior to ovulation and 3 weeks after the last time we made an attempt to conceive, a passing familiarity with human conception will tell you that this is impossible. No, in fact, this baby was conceived when I actually ovulated and not by some freaky and convoluted miracle process of my dear husband’s swimmers breaking all records and my body releasing an egg more than a week before all the physical signs indicated the possibility.

Even so, knowing when you conceived doesn’t actually give you any particular insight into how long your pregnancy will turn out to be. Gestation is a natural process and as such, it can’t be timetabled. There is great variation and most of it is completely incomprehensible, there is no known explanation for the wide variety in gestation lengths, it’s just an organic part of life and one that I think we would all do well to accept.
People pin all of this importance on the EDD and their care providers make it worse by rushing to intervene as soon as pregnancy continues beyond 40 weeks, which it does in more than 50% of all pregnancies! In my own small way, I try to challenge this by reminding people of the “Estimated” in EDD and emphasising that normal pregnancy is 37-42 weeks. I have also refused to tell anyone the precise date that I will be 40 weeks pregnant. I know when that date is, as I know when we conceived, but it is not my EDD, to me there simply is no such thing as an EDD. Our midwife knows the 40 week date, but together we have no expectation that there is anything special about that one day. It’s a vaguely useful day to note down so that we have an idea of what 5-6 week period the baby might put in an appearance, that’s all. To all of our friends, family, neighbours and random acquaintances, we expect this baby to arrive some time in February, probably.