Ovulation usually occurs about two weeks after your menstrual period and lasts for two or three days. This means you can sometimes determine when you conceived based on your ovulation cycles — simply determine when your last menstrual period was, and add approximately two weeks.
However, this method isn’t always accurate. Not all women have the same cycles of menstruation and ovulation. Some women’s menstrual cycles are irregular. Others have ovulation periods closer to or further from their menstrual period. Some women simply aren’t sure when their last period was. In other words, calculating conception based only on your body cycles can be difficult.
This can make it difficult to determine when you conceived, especially if you have had more than one partner during the time when you ovulated. Also, remember that you could become pregnant even if you or your partner used birth control, since no method of birth control is 100% effective at preventing pregnancy.