Everybody knows that the average pregnancy lasts 9 months. However, those of you with kiddos are probably aware that things take 40 weeks to reach term. Now, a little simple math makes it obvious that those can’t both be true. So what gives?
So, here’s the hypothesis we’re currently working on over here in Ob/Gyn land (sad, I know, that a bunch of medical types are turning to the net for medical info, but I figure you all owe us one): back in the day (and even now, for those who aren’t trying), a lady wouldn’t know she was pregnant until she missed her period, which classically occurs 14 days after ovulation/conception. So…40 weeks - 2 for mom to clue in to the arrival of the new parasite = 38 weeks. And the 40 weeks that docs like to talk about is actually dated from the last known menstrual period (eg, 14 days on the other side of ovulation/conception), so that 40 should be minus 2 to account for that, also yielding 38 weeks. Which is all nice and happy, and seems reasonable.
But we’ve got another thought. Back in the day, when the whole “9 months” was ingrained as a cultural concept, nutrition probably wasn’t so hot for the average mom, and the pregancies probably were therefore a bit premature compared to the current standard. Which could also subtract a few weeks off of the 38 (40 - 2 for the fact that we date from last known menstrual period), giving 36 = 9 months.
So, basically, we came up with two good hypotheses for one bad question. Which is why we’re turning to the teeming millions (at least a few of whom, one hopes, majored in history of science, and can look this up in a book).
Thanks in advance
-ellis
Addendum question: Is 9 months a purely western cultural concept? Do the ladies over in Tokyo and Timbuktu expect their kids to pop out in 9 months/40 weeks, too?
But 9 months != 36 weeks. 9 months = 39 weeks, give or take a day or so. Which doesn’t necessarily invalidate your question, but does make the discrepancy much smaller.
I’d thought that average term was anywhere from 38-42 weeks, in a sort of bell curve around 40 weeks. I doubt that many women go beyond 40 weeks anymore since OB/GYNs are often in a hurry to induce or slice, though. And of course for most couples the date of conception is an inexact science. For those who are positive, as in cases of in vitro, then they are less likely to go beyond the OB/GYN’s comfort zones anyway.
If my range is roughly correct, that puts from 8.77 months to 9.70 months as average. So it fits nicely in the “conventional wisdom” window. How are you med folks at math?
Not very good, evidently. We were happily working with 1 month = 4 weeks. I blame it on sleep deprivation.
Thanks for the link, antechinus; I don’t quite follow the implication of the other thread that the Mittendorf study hasn’t been accepted, though. By my (poor) math, an average gestational period of 274 days ~ 39, which, as has just been pointed out to me, is pretty spot on with the traditional 9 months; this also makes the 40 weeks taught by current OBs a little generous, if anything. Naegel might have been off, but I think 40 weeks has been the standard for a while.
This last sort of touches on the “slice and dice” view of modern OBs. Nobody is eager to let a lady go past dates, but nobody pulls out a knife as the clock ticks 40. Along with the wiggle factor that Mittendorf demonstrates, studies have shown that 34 weekers have equivalent morbidity and mortality data (and that 32 weekers only suffer from a slighter longer average intial post partum stay, but no increase in mortality or major brain injury, etc.). Caveat must be given that I haven’t read the studies myself, but the texts and teachers are all solid on it, and it’s certainly data that protocols are built around. Point being, you’d have to be pretty wildly off in your dates to cause any harm by inducing at 40 weeks.
Sorry, I was WAY off. 1/3 = .3333333etc… I should have used 4.3333333etc. instead of 4.3 and I would have come up with 38.999999 weeks, 4 hours, twenty seven minutes and thirty two seconds.
I apologize profusely.
I can’t believe that no one has mentioned this yet, but the 40 week standard starts at the date when the woman had her last period. Pregnancy itself occurs about two weeks later.
