Seahorses are the only animals where males are said to get pregnant
In seahorses, pipefish and sea dragons, the female deposits her eggs into a pouch on the male's chest, and he carries them until they hatch. It is the only animal family for which scientists use the phrase male pregnancy, and one striking twist on gestation, the time offspring spend developing inside a parent.
Gestation belongs to viviparous animals, whose embryos grow inside the parent rather than in an external egg. Mammals are the classic case, but placental live birth also evolved separately in scorpions, some sharks, velvet worms and certain lizards, such as the genera Tiliqua and Corucia, whose placenta connects directly to the mother.
A halfway strategy is ovoviviparity, used by most vipers. Eggs stay inside the mother until they hatch or nearly do, but the embryos live off yolk rather than being fed by her body, though she still supplies gas exchange. Some amphibians born this way emerge as larvae and finish metamorphosis outside.
Length of gestation seems tied to lifespan. Across 740 mammal species, longer gestation correlated with longer maximum lifespan. During pregnancy in placental mammals, cells of the placenta and the uterine lining it anchors to gradually accumulate DNA damage and age, and researchers have proposed that rates of such damage might influence both how long gestation lasts and how long an animal lives.
For humans, a pregnancy is conventionally split into three trimesters of about three months each, and birth typically arrives around 40 weeks, commonly anywhere from 37 to 42. Doctors date it from the first day of the last menstrual period, about two weeks before fertilisation. Biologists instead count from implantation of the fertilised egg, and a biochemical definition uses a hormone threshold, so the start of pregnancy depends on who is measuring.
Source: Gestation
