Pregnancy is the carrying of one or more offspring, known as a fetus or embryo, inside the uterus of a female human. 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. Obstetrics is the medical field that studies and treats pregnant patients.Childbirth usually occurs about 38 weeks from fertilization, i.e., approximately 40 weeks from the start of the last menstruation. Thus, pregnancy lasts about nine months, although the exact definition of the English word “pregnancy” is a subject of controversy.One scientific term for the state of pregnancy is gravid, and a pregnant female is sometimes referred to as a gravida. Both words are rarely used in common speech. Similarly, the term "parity" (abbreviated as "para") is used for the number of previous successful live births. Medically, women who have never been pregnant are referred to as "nulliparous" ("gravida 0, para 0"),, during a first pregnancy as a "primigravida" ("gravida 1, para 0") and in subsequent pregnanies as "multigravida" or "mutliparous". Hence during a second pregnancy a woman would be described as "gravida 2, para 1" and upon delivery as "gravida 2, para 2". Incomplete pregnancies of abortions, miscarriages or stillbirths account for parity values being less than the gravida number, whereas a multiple birth will increase the parity value.The term embryo is used to describe the developing human during the initial weeks, and the term fetus is used from about two months of development until birth.In many societies' medical and legal definitions, human pregnancy is somewhat arbitrarily divided into three trimester periods, as a means to simplify reference to the different stages of prenatal development. The first trimester carries the highest risk of miscarriage (natural death of embryo or fetus). During the second trimester, the development of the fetus can be more easily monitored and diagnosed. The beginning of the third trimester often approximates the point of viability, or the ability of the fetus to survive, with or without medical help, outside of the uterus.