Pregnancy is the fertilization and development of one or moreoffspring, known as an embryo or fetus, in a woman's uterus. It isthe common name for gestation in humans. A multiple pregnancyinvolves more than one embryo or fetus in a single pregnancy, suchas with twins. Childbirth usually occurs about 38 weeks afterconception; in women who have a menstrual cycle length of fourweeks, this is approximately 40 weeks from the start of the lastnormal menstrual period (LNMP)