Due date calculator

In order to determine your due date, they generally
start counting on the first day of your last menstrual
period. If you happen to know when you ovulated (e.g., if
you were charting), then you could use that date instead
and it would likely be more accurate.


But since it appears
you weren't, the LMP is the date they'll go with. If you
don't know that date, or if your cycles are very irregular
they may send you in for a dating ultrasound after your
first visit. Otherwise, your LMP date will get you in
the ballpark.


Since your due date calculator is only good plus or
minus a couple of weeks anyway, it's not essential to be
absolutely precise (babies come when they're going to come
anyway). The only caveat is that if your doctors are
going to start pushing for an induction as soon as your
due date comes and goes (most first timers go past their
due date - the median length of gestation for first timers
is 41 weeks 1 day), then you may want an accurate date
so that you know they're not going to be pushing for
induction thinking you're post due when you really aren't.


Early ultrasounds are very, very reliable for determining
the gestational age of a fetus. They get unreliable by the middle or the
second trimester because there's more variation in growth rates between
individual fetuses, but early in pregnancy and especially in the first
trimester they are virtually spot on. I would believe the ultrasound date
over your own date based on LMP or other calculations. It's much more likely
to be correct.

If you do a search on the web for "due date calculator"
you will turn up several sites where you can enter your
LMP date and they will give you your due date. Or you
can do the math yourself and count 40 weeks from your LMP
date.