Last menstrual period: By convention, pregnancies are dated in weeks starting from the first day of a woman's last menstrual period (LMP). If her menstrual periods are regular and ovulation occurs on day 14 of her cycle, conception takes place about 2 weeks after her LMP. A woman is therefore considered to be 6 weeks pregnant 2 weeks after her first missed period.
A woman's obstetric date is different from the embryologic date (the age of the embryo). The obstetric date is about 2 weeks longer than the embryologic date.