The most recent menstrual cycle Add 7 days and then subtract 3 months. If your previous period began on March 20, you would add 7 days to get March 27. Then subtract 3 months to obtain a December 27 due date. Add 40 weeks to the beginning day of your last period to get an idea of when you'll be due. (The first day of your pregnancy is called "conception," so start counting at this time too if you aren't already pregnant.) It's best to wait until after the first trimester to find out your due date. A doctor can estimate it for you using just your last menstrual period and your height without measuring the size of your belly. But don't worry about guessing early; it isn't accurate enough for that purpose. The first sign that you're near your due date is feeling mild cramping in your abdomen. As the date gets closer, these pains will come more frequently and last longer. You may also experience mood changes, including irritability or depression. At the end of the third month, your baby should arrive anytime from there on.