Estimated Due Date Demystified

So these days whenever Shu or I bump into any friends or co-workers on the street or at the water cooler we are met with big eyes and cries of near alarm that the baby hasn't arrived yet! This is because all along we've been tracking for an Estimated Due Date (EDD) of 5/19, which was 7 days ago. Well first of all the EDD is just that, an "estimate", and in fact only 5% of babies are born on the due date but 80% are born within +/- 10 days of the due date. Secondly, the questions of whether baby is late and when to become concerned and think about inducing are based on a premise that we have a reasonable EDD.

As it turns out there are different ways to estimate gestational age of the fetus and consequently the EDD:

1) Using an ultrasound to measure fetus size and estimate date of conception
>> See Calculating Your Dates: Gestation, Conception & Due Date

2) Computing from first day of mother's last menstrual period (LMP)
>> See How to Calculate Your Pregnancy Due Date

In our case, our OB performed the first ultrasound on 10/14/10 and measured the fetus size. With this measurement she could then estimate both date of conception and a resulting EDD of 5/19/11.

However, using the second LMP method above we independently determined based on an estimated LMP of 8/19/10 a reasonable alternative EDD of 5/26/11, which is today!

(In fact, at the time of the OB visit on 10/14/10 we cluelessly failed to tell our OB the LMP date of 8/19/10, so she reasonably used method #1 above to arrive at the 5/19/11 due date.)

So we can all rest easy now knowing that 5/26/11 is just as reasonable a due date and a delivery up to 10 days beyond that (by 6/5/11) would still fall within 80% of band of pregnancy terms.

OK, now go back to sleep!
Paul & Shu

p.s. The picture above is of all parents earlier tonight. Yes, the parental units are now all in town and on at the Orange Alert level!