Human pregnancy is the carrying of one or more offspring, known as a fetus or embryo, in the womb of a woman. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets. The World Health Organization defines normal term for delivery as between 37 weeks and 42 weeks. Human pregnancy is the most studied of all mammalian pregnancies. This calendar contains the entire cycle of human pregnancy and all major events within a series of colored circles.