Questions & Answers

An early ultrasound is more accurate if you have no idea when you ovulated. So if your ultrasound EDD does not match up with the first day of your last period, your doctor will probably go with the EDD based on your ultrasound.

If you know when ovulation was (or in my case egg collection) you can count back 14 days to get a rough LMP. But that's only rough. I know exactly when my 5 day embryo was placed in my uterus. You would have thought that this would give pinpoint accuracy. But it could have implanted any time between Day 6 and Day 12. It's the same for any woman. There's a six day window. However, the sonographer can work out exactly how pregnant you are, based on the length from crown to rump, whether a heartbeat is visible, the size of the amniotic sac and the presence/ size of a yolk sac. I think I know exactly when my embryo implanted (the morning of day 6) because I felt it happen. The HcG levels and the ultrasound back this up. :-) However, like you, I don't really have an accurate LMP because my cycle was controlled by medication. Go with the dating offered by your sonographer. It's very accurate.

With or without a period, many doctors change or confirm a due date based off of the ultrasound. Periods only give an estimate, but it varies even with a period so ultrasound is more accurate.