The egg that will become the baby rapidly divides into many cells within 24 hours of fertilisation. The embryo grows into a foetus by the eighth week of pregnancy. A typical pregnancy lasts roughly 40 weeks. The actual embryo or foetal age (also known as conceptual age) is the amount of time that has passed since the egg was fertilised near the time of ovulation. However, because most women do not know when ovulation happened but do know when their last period began, the time passed since the first day of the previous normal menstrual cycle, known as the menstrual age, is used to calculate the age of a pregnancy. The gestational age is another name for the menstrual age. Traditionally, gestational age is represented in completed weeks. As a result, a 36 week, 6 day foetus is treated as a 36 week foetus. Prenatal development, or development before birth, is described in a variety of ways. The phrase "antepartum" has the same meaning (from Latin ante "before" and parere "to give birth") However, the term "antepartum" is sometimes used to refer to the time between the 24th and 26th week of pregnancy and birth, as in antepartum haemorrhage. The perinatal phase (from Greek peri, "around" and Latin nasci, "to be born") is defined as "the time leading up to birth."