Diet and Health FAQ

Tagged As: Pregnancy Due Date

Question:
Does the number of days in a woman's cycle affect the due date. Most pregnancy due date calculators I have seen assume a 28 day cycle. Some I have seen allow you to enter cycle length. Does it really matter?

Answer:
It can do if you have a particularly short or long cycle. One of my friends has a cycle that is never longer than 21 days, and I know there are women who have cycles of 35 - 40 days too. Those extra days can make a difference to the due date, which is a little more important if doctors start making a fuss about the fact that you've gone over your 40 weeks. Yes! Most women will ovulate 12-16 days *before* the start of their period. Its the length of time before ovulation which usually varies, not after. The average cycle is supposed to be 28 days, but anywhere from 21 to 35 is pretty normal. Most pregnancy calculators use the first day of the last period as their start date and assume ovulation on day 14. If you actually ovulated on day 28, you are two weeks less pregnant than you thought! If you have an induction happy OB, you might find yourself induced before your real due date. Believe me, I ovulated on day 31 when I got pregnant - its worth battling at teh beginning to get the date calculated correctly. I'd be off to my 41 week appointment this week and declining induction had I not got it corrected. I'm actually not due for another 10 days. Your due date is 266 days after your ovulation date, which generally occurs about 14 days before the last day of your cycle. So most calculators, assuming a 28 day cycle, are assuming ovulation on day 14 and therefore just add 280 to the first day of the cycle. This estimate will be a few days off if your cycle is more or less than 28 days. If you want a more accurate guess, you can subtract 14 from the day you expected your period, then add 266 days to that date. As others have said, it can matter a *lot*. Ovulation occurs 12-16 days before conception, regardless of how long your cycle is. Assuming a 28-day cycle can result in setting a due date that's either too early or too late (though I'd much rather have a too late EDD than a too early one!). The assumption of a 28-day cycle is particularly troublesome for women like me who have very long cycles (mine are 35-45 days), since it can result in setting an EDD that is as much as 2 weeks too early. With every one of my pregnancies, my original EDD calculated by my practitioner on the blasted wheel was *way* off. With my third, for example, my due date by LMP and the wheel of fortune was 2/18/02; I knew that was well off, and an ultrasound caused that date to be bumped to 2/28/02. All I can say is that I am very thankful that early ultrasound confirmed my dating, because I'd have wound up with way too early induction for being post-dates without it.