The only way you can be fairly certain of the exact day you became pregnant is if you only had one sexual encounter during your fertility window. This can be anywhere from 11-21 days after the first day of your last period, and you know the exact day that encounter took place.

How do you find out what day you got pregnant?

Thats the day you conceive. So without knowing the day of conception, how does anyone determine a due date? The most common way to calculate your pregnancy due date is by counting 40 weeks from the first day of your last menstrual period (LMP). And thats how most healthcare providers do it.