This is traditionally calculated as the number of weeks since the first day of an expecting mother's last menstrual period. In evaluating expected development of the baby for a point in pregnancy, this dating method assumes that the mother has a 28-day cycle with ovulation on the 14th day of the menstrual cycle. The average pregnancy with this calculation system lasts about 40 weeks, from the start of the last menstrual period until birth.
If ovulation does not occur on the 14th day after the start of the last menstrual period, such as for women with irregular menstruation, the development of the pregnancy may not exactly match the expected development counting from the last menstrual period. For example, an ultrasound 7 weeks after the last menstrual period may show a 6-week gestational age if the mother has 35-day menstrual cycles with ovulation on the 21st day after the menstrual period starts.