Pregnancy is the carrying of one or more offspring, known as a fetus or embryo, inside the womb of a female. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets. Human pregnancy is the most studied of all mammalian pregnancies. Childbirth usually occurs about 38 weeks after conception; in women who have a menstrual cycle length of four weeks, this is approximately 40 weeks from the last normal menstrual period (LNMP). The World Health Organization defines normal term for delivery as between 37 weeks and 42 weeks.