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Human pregnancy can be divided into three trimesters, each approximately three months long: the first, second, and third trimester. The first trimester is from the last menstrual period through the 13th week, the second trimester is 14th–28/29th week, and the third trimester is 29/30th–42nd week. [ 6 ]
Pregnancy is divided into three trimesters, each lasting for approximately three months. [4] The exact length of each trimester can vary between sources. The first trimester begins with the start of gestational age as described above, that is, the beginning of week 1, or 0 weeks + 0 days of gestational age (GA).
A human fetus, attached to placenta, at three months gestational age. In humans, the fetal stage starts nine weeks after fertilization. [7] At this time the fetus is typically about 30 millimetres (1 + 1 ⁄ 4 in) in length from crown to rump, and weighs about 8 grams. [7] The head makes up nearly half of the size of the fetus. [8]
In response to the conditions of the environment, some mammals, such as bat delay the implantation due to the cold temperature in winter. [23] Another factor is due to the shortage of food stocks during winter as the insects are being driven away and as the result, bat hibernate in pregnant condition. [24]
Weeks and months are numbered by gestation. Different terms are used to describe prenatal development , meaning development before birth . A term with the same meaning is the "antepartum" (from Latin ante "before" and parere "to give birth") Sometimes "antepartum" is however used to denote the period between the 24th/26th week of gestational ...
It is rare for a baby weighing less than 500 g (17.6 ounces) to survive. [13] A baby's chances for survival increases 3–4% per day between 23 and 24 weeks of gestation and about 2–3% per day between 24 and 26 weeks of gestation. After 26 weeks the rate of survival increases at a much slower rate because survival is high already. [15]
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During vaginal birth, the newborn's chest is compressed by the birth canal. Upon delivery, negative pressure allows air into the lungs. The first cries of the infant allow for alveoli expansion and absorption of fetal lung fluid. Temperature changes and other sensory stimulation contributes to respiratory function as well.