I’m writing a story with a friend for kicks, but since people are actually reading it, we’d like to be as accurate as possible. Sadly, neither of us quite understands the nine months but ten months deal.
If our character is going to concieve a baby the first week of January would the baby be due… early or late October? Does adding forty weeks to whatever the conception date is give a good estimate of when a baby is due, or is it harder to work out than that?
I actually researched this a few months ago and posted in this thread.
A paper (AM J OBSTET GYNECOL 1993;168:480-4) was published in 1993 by Mittendorf et al, which statistically analysed a sample of deliveries (n=9355) and came up with an algorithm to more accurately predict delivery date.
GL = 256.447 + 1.5648age - 0.0259age^2 + 0.1295*ponderal index
Much better than the 200 year old Naegels rule, that is commonly used.
The “typical” pregnancy supposedly lasts 280 days – ten lunar months of 28 days, or nine calendar months of 30-31 days, plus a smidge. Obviously folks have already given more precise modes of calculation that take individual differences into account – but that’s the sense behind the nine/ten month dichotomy.
Another aspect of this is the calculation of the start of the pregnancy, which is considered to be the end of the woman’s last period. Given that the majority of women ovulate roughly 2 weeks after this, it can get confusing. Let’s say your characters had intercourse only one time, on January 3rd. The actual “date of conception” would be from the end of the female character’s last period, most likely roughly 2 weeks before the act (say December 21). Something to consider if factual accuracy is your ultimate goal.
There are also real lunar months (29.53 days) and fudge lunar months (28 days).
Polycarp The ten lunar month thing is the time from the first day of the mothers last menstrual cycle, which means the actual gestational period is ~ 10 x 28 minus two weeks for the time between menstruation and ovulation = 266 days. (according to this method of estimation).
But of course average gestation is closer to 274 days (based on empirical evidence of the Mittendorf study).
oh and of course the 2 weeks from menstuation to ovulation can vary quite a bit.
So using the rough and ready method for birth date:
From Menstruation
10.0 fudge lunar months
9.5 real lunar months
9.2 calander months
From Conception
9.5 FLM
9.0 RLM
8.7 CM
Using 274 day gestation from conception:
9.8 FLM
9.3 RLM
9.0 CM
Thanks! Follow up question though: If a premie is a “24-weeker” do they mean the baby has been developing for 24 weeks, or mom’s last period was 24 weeks ago? Because if it’s the latter…wow.
I think that’s gestation time on the preemie. But is a 24 week preemie viable?
When I was having a hospital tour before the birth of my baby, there were pictures up on the walls of prem babies who had previously been born there (and survived). The youngest was 23 weeks.
My parents told me i was born at 7 months. I was born at 8 pounds though. Is that likely or even possible by any of these calculations?
I like to use weeks for accuracy. 38 to 42 weeks is considered ‘term’, though most Doctor’s won’t let you get to 42 weeks.
I have a son born at 23.5 weeks who is 6, in kindergarten, and without apparant deficits. We’ve been pretty fortunate.
Where I work, we’ve had three 22 week survivors over about 6 years, but I don’t know how long two of them survived or how they are doing. I did see one, who was only 8 months old at the time. She still had apnea, oxygen, tube feeding, and appeared to have significant brain damage, although I did not ask the mother about her.
Depends on your parents. Are you a first-born? Here’s what I mean-
In my husband’s family, there has always been a joke about 1st and 2nd babies. They say that first pregnancies can be any length, and subsequent pregnancies are 9 months.
In case it’s late where you are, it was probably to cover up marriages caused by pregnancy.
If they were serious, holy cow. 8 weeks premature and 8 pounds? You might have killed your mom! 
True. If you count back seven months from your birth and get your parents’ wedding day, odds are somebody is fudging the numbers a bit.
Not that it’s that big a deal. I’ve read about some Scandinavian cultures where a couple would live together and only marry if/when the woman’s pregnancy was detected, thus demonstrating the union would be productive.
I don’t know why not. Of course a lot of first-born children were often called “preemies” back in the 50s and 60s, even though they were 9 lb. and 21 inches long! Gee, I wonder why?
Actually, when I gave birth to my son, the baby born to the couple across the hall from me on the maternity ward was a whopping 14+ lbs., and he was a term baby. He looked like a bouncing baby watermelon, he was just huge!
14 pounds?!? My uterus hurts just thinking about it!
Mothers with gestational diabetes often have larger babies.
This may have been her case and she didn’t know it. I don’t think they tested for it in the past.
Well, you could practice not splitting the infinitive the next time around.
“Babies are carried for nine months. Or ten. Care to help out two childless writers?”
Actually, the sentence was ended in a preposition.
Splitting the infinitive would be “Care to, two childless writers, help?”
Also popularly known as Yoda-speak.