- published: 07 Nov 2015
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Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time (multiple gestations).
The time interval of a gestation is called the gestation period. In human obstetrics, gestational age refers to the embryonic or fetal age plus two weeks. This is approximately the duration since the woman's last menstrual period (LMP) began.
Human pregnancy can be divided into three trimesters, each three months long. The first trimester is from the last period to the 13th week, the second trimester is from the 14th to 27th week, and the third trimester is from the 28th week to 42 weeks.
In humans, birth normally occurs at a gestational age of about 40 weeks, though a normal range is from 37 to 42 weeks. (about 9 months)
In mammals, pregnancy begins when a fertilized zygote implants in the female's uterus and ends once the fetus leaves the uterus.
Below are average and approximate values ordered by gestation period (note for humans gestational age is counted from the LMP, for other animals the counting method varies, so these figures could be 14 days off):