Hi.


I did an ultrasound at 3 Oct and it said I was 5w5d pregnant. My LMP was on 28 Aug. I am an average cycle of 26 days. With my longest cycle being 28 days. Is it possible to determine the conception date from the ultrasound? And how accurate will it be? Can anyone tell my what my possible conception date was?

Gestational dating is an inexact science based on mathematical formulas. Since you don't know exactly when you ovulated, it is a guessing game based on that formula. Gestational dating has a two week + or -. Therefore, if paternity is in question, you will have to have a DNA test after baby is born.

Yes, give or take two weeks is about correct with the dating.

The dating from ultrasound is based on average sizes, so many babies larger, many smaller, some right on average.

The idea of an "exact" day is just not possible. Is there a reason for concern?

Dating on an ultrasound is done by development of specific organs and length of certain bones such as the femur. Fetal organs develop at different times during pregnancy. The firs organ that develops is the heart, and so on. Dating by ultrasound is determined by the development of the fetus. Not by size of the fetus alone.