Dating pregnancy irregular periods
My last period was 20th March but I'm only 12 6.
There are two common ways to date a fetus- one is going off your last period, the other is going off the conception date.
You can think of fetal age as really like egg age, since those first two weeks happen before any sperm comes into contact with the egg while the egg travels down the Fallopian tube.
But that tells you how old the fetus is, not how far along in the pregnancy you are, since you're considered pregnant 2 weeks before conception.
I am actually not pregnant, but I know someone who is and this stuff become fascinating to me. They'll also measure the baby at some point and give another estimation of due date to see if it's around a week of the LMP prediction.
When they use the conception date, they assume you are 2 weeks pregnant at conception, even if you didn't have a period 2 weeks ago.
I have super irregular cycles, like 40-80 days compared to the normal 28.
I have pcos and dodgy periods so I was very curious about the same thing.
The way the tech explained it to me was that the measurements are matched to the average crown rump length and the date given would be as if your lmp date were later.