At the hospital I work at as a high-risk labor and delivery RN, the docs consider 40 weeks full term and the sonogram is the end-all-be-all of due date forecasting. Dr. F. won’t go one day past the due date on a gestational diabetic mom—she induces them even if the baby isn’t really that large and has been sending them for NSTs every few days. Dr. H will induce his moms at 36.5 weeks if their blood pressures have been high and they’ve shown protein in the urine. He sends them for NSTs almost daily. So does Dr. M-W if her mom has any history of fetal demise. I monitor fetal heart tones and mom’s contractions externally on every mom from the moment she gets admitted until she delivers, per department protocol. I check the FHTs for 20 minutes for reactivity and evaluate the cervical dilation myself before I start an induction, no matter what doctor ordered induction for whatever reason.
Cyn, RN
They’re called lunar months (29.5 days) and ‘lunar months’ (28 days). 
But that 40 weeks is calculated from the start of the last menstrual period, so on average the conception is two weeks later–that is, a gestation period of (40x7 - 2x7) 266 days.
Thanks for all the replies guys.
RM Mentock - the point is the average gestation period is not 266 days it is 274 days. My wife and I actually went for a small walk from the hospital (where she is now - and being induced with prostoglandin - I will be there in a few hours, or earlier if a call comes) to the Faculty of Medicine library and copied the 1990 paper From the Journal of Obstetrics and Gynacology, that determined this average empirically.
I questioned 2 midwives - they basically said 266 days is done for convenience. That way they dont have to introduce other variables - such as primiparity and race into an algorithm. Even though it was invented by some moon-gazer back in 1812.
Happy birthday antechinus!
I went through it 3 or 4 times, and it focusses your mind incredibly. I’m sure everything went well with yours. What’s her name?
Try not to dwell on the possibility that your baby was cheated out of two days in the womb. Babies have been known to be born months early and grow up with no adverse effects whatsoever, size or health. A couple of days means almost nothing, as near as I can tell.
It drove me crazy too. Why ten ‘months’? Why the first day of the last period–if you know the conception date? What are the “error bars”? Bottom line is that a couple weeks makes almost no difference–except to the mother, who is usually anxious just to get it over with. So be sympathetic.
Thanks for bringing this up, I hadn’t heard about the Mittendorf study. I notice from recent stuff that I just gleaned off the internet that Mittendorf’s work is being refined. One thing that I noticed when we went through it was that the bone measurement recalculation (which someone else has already mentioned as being the best predictor) of due date, for us, was uncannily (within a day) close to 266 days of conception. So, no one ever knows for sure. That’s life.
Good luck.
Dude, get off the computer and over there! This is a first-time experience for her, a very powerful psychosexual experience that she’s been told all her life is going to be unbelievably painful. Not only that, now she’s alone in a hospital with an IV. She needs you.
On the other hand, it may be a while until she actually gets going. I took about four hours. When my husband ducked out to get lunch, he went to the used bookstore next door and brought me back a couple new books. It was terrific to have something to take my mind off things.
Good luck to both of you!
HennaDancer
Congrats antechinus
Aint it soo cute watching someone deal with new baby jitters by analyzing it to death…