We measure a pregnancy from the time the ovum begins to grow, that is, at the start of a woman's menstrual period. After about two weeks of growth, the egg is released from the ovary. Fertilization can then occur. This is about two weeks before her next period is due. Four of the 40 weeks have already elapsed at the time she misses her first period.
Gestational age dates from the first day of the mother's last menstrual period.
Actual age of the baby dates from conception. Birth is the emergence of the infant from the mother's womb, the severing of the umbilical cord, and the beginning of the child's existence physically detached from the mother's body. The only change that occurs at birth is a change in the external life support system of the child. The child is no different before birth than after, except that he has changed his method of feeding and obtaining oxygen. Before birth, nutrition and oxygen were obtained from the mother through the baby's umbilical cord.