Within 24 hours of fertilization, the egg that will develop into the baby rapidly divides into several cells. A fetus develops from the embryo by the eighth week of pregnancy. Usually, pregnancy lasts for 40 weeks. The length of time since the egg was fertilized close to the time of ovulation is the real embryonic or fetal age, often referred to as conceptual age. The menstrual period—a measure of time since the first day of the previous regular menstrual cycle—is used to determine the age of a pregnancy. At the same time, most women do not know when ovulation occurred but when their most recent period started. Menstrual age is often referred to as gestational age. The traditional way to express gestational age is in completed weeks. As a result, a 36-week, 6-day-old foetus is handled similarly to a 36-week-old fetus. There are many different methods to define prenatal development or development before birth. The word "antepartum" has the same connotations. The term "antepartum," however, is occasionally used to refer to the period of time between the 24th and 26th week of pregnancy and delivery, as in the case of an antepartum hemorrhage. "The period preceding birth" is referred to as the perinatal phase.