Most pregnancies last around 40 weeks (or 38 weeks from conception), so typically the best way to estimate your due date is to count 40 weeks, or 280 days, from the first day of your last menstrual period (LMP). Another way to do it is to subtract three months from the first day of your last period and add seven days. So if your last period started on April 11, you’d count back three months to January 11 and then add seven days, which means your due date would be January 18. Use this pregnancy test calculator to find your due date.
Conception usually occurs around 11-21 days after the first day of the last period of a woman who has a regular period. The estimation of conception date is based on this but is rarely ever exact since it is difficult to know exactly when ovulation occurs.