How Many Weeks?
Refers to the time elapsed since the mother’s first day of her last menstrual period (LMP). Typically, this is two or more weeks earlier than the time of actual fertilization. If a woman had her period two weeks previously and then conceives, the gestation age of the developing baby is two weeks. The gestational age is the one traditionally used by medical providers to date pregnancy.
Refers to the time elapsed since conception: the fertilization of the egg by the sperm. Most women ovulate (release a mature egg from the ovary) in the middle of their monthly cycle. For example, if a woman has a 28 day cycle, she will typically ovulate on the 14th day after her period began. If a woman conceives and becomes pregnant at that time, this becomes the fertilization day.