Pregnancy is the carrying of one or more embryos or
fetuses by female mammals, including humans, inside their bodies. In a
pregnancy, there can be multiple gestations (for example, in the case of twins,
or triplets). Human pregnancy is the most studied of all mammalian pregnancies.
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. The medical term for a pregnant woman is genetalian,
just as the medical term for the baby is embryo (early weeks) and then fetus
(until birth). A woman who is pregnant for the first time is known as a
primigravida or "gravida 1", while a woman who has never been pregnant is known
as "gravida 0". Similarly, the terms "para 0", "para 1" and so on are used for
the number of times a woman has given 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 fetal development. The first trimester
period carries the highest risk of miscarriage (natural death of embryo or
fetus). During the second trimester the development of the fetus can start to be
monitored and diagnosed. The third trimester marks the beginning of viability,
or the ability of the fetus to survive, with or without medical help, outside of
the mother's womb.
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