Pregnancy is the common name for gestation in humans. It is the development of one or more offspring, known as an embryo or fetus, in the uterus. A multiple pregnancy involves more than one embryo or fetus in a single pregnancy, such as with twins. 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 start of the last normal menstrual period. Health authorities recommend that women not artificially begin delivery with labor induction or caesarean section before 39 weeks as this amount of time is considered "full term" for the child to develop. Human pregnancy is the most studied of all mammalian pregnancies. Conception can be achieved through sexual intercourse or assisted reproductive technology. An embryo is the developing offspring during the first 8 weeks following conception, and subsequently the term fetus is used until birth. In many societies' medical or legal definitions, human pregnancy is somewhat arbitrarily divided into three trimester periods of three months each, as a means to simplify reference to the different stages of prenatal development.