Just wondering about those online due date calculators. Are they at all accurate? Did anyone play around with one when you found out you were PG and did your doctor give you a wildly different due date, or were they pretty close?

I think they just measure 40 weeks from your last period (LMP), which is exactly what your doctor does - unless an ultrasound shows you measuring less/more than that.  Since I have slightly longer cycles, I'm due a few days later than what comes up on an online calculator.

They assume a 28 day cycle.

DS was born the exact date of those online calculators. He avoided all the dates the u/s predcited.

They all make the estimate based on the date of your LMP.  They do this because given that sperm can live up to 5 days waiting around for you to ovulate, even if you only had sex one time that month, you still can't know for sure what day fertilization occurred.    And, gestation time is an average for human beings not an exact amount of time every baby will spend in the womb.  So between not being able to determine when implantation occurred and the fact that not every baby spends the same amount of time in the womb, LMP date gives you a very good, easy to calculate estimate.

Later, the due date might be revised based on actual fetal measurements.

They will use your last LMP to give you and EDD.  This can change however, especially with very early ultrasounds (for example, this put my EDD back one week).  My OB said that the early u/s is more reliable for determining the date than later ones, as when they are early, there is less variation of growth rates etc.

Some online calculators will have a place to input your cycle length if it is regular, but longer or shorter than 28 days (the default for most calculators.) I played with a few, They all gave dates within a week of the one my OB gave, and my DD was born 2 days after the date my OB gave. The majority of babies are not born on their due dates, but a majority are born +/- a week of the due date given by the doctors.

In Response to Re: online due date calculators:


I'm glad you wrote that :o)
LMP would be 2/19, but my first ultrasound indicated 2/22 as the due date.  That is the date I prefer!  I only JUST found out that the next u/s indicated 2/24 and that's the date the doctor has been using... without telling me!  As far as I knew she was still using 2/19.

Glitzy - it's 100% accurate in determining 40 weeks out on the calendar from the date of your LMP.  But anyone who can count to 40 can do that with the same accuracy...  However, if you ovulate late like I did (on day 20 vs. day 14), then I'd trust the dating of the first u/s instead.  But it's accurate in getting a good general idea of when you'd be due.  And even then, it's up to the baby to decide :)