Elephants carry a fetus for 23 months; jumping mice need just 18 days
A fetal stage lets animals give birth to young that are already well developed, at a real cost to the mother. How long that takes varies enormously: placental mammals carry young for anything between 18 days, in jumping mice, and 23 months, in elephants. In general, the bigger the land mammal, the longer its fetus needs.
A fetus is the unborn offspring of a live-bearing animal in the stage between embryo and birth. There is no sharp line between the two, since development is continuous, but a fetus generally has all its major organs in place, even if they are immature and some have yet to settle into their final positions. Fetal stages occur in most mammals and many fish, and among invertebrates in some onychophorans and many arthropods.
In humans the fetal period begins nine weeks after fertilisation, equal to eleven weeks of gestational age. At that point the fetus is about 30 millimetres from crown to rump and weighs roughly 8 grams, with the head making up almost half its size. Its heart, brain, hands and feet exist but barely function. Breathing-like movements help the lungs develop rather than supply oxygen. By the end of the fifth month it is about 20 centimetres long, and first-time mothers usually feel movement around week 21.
Late pregnancy brings a rapid build-up of fat, bones that are complete but still soft, and fingernails reaching the fingertips. Links between the sensory cortex and thalamus form as early as 24 weeks but show the first signs of working only around 30. Survival exceeds 90 percent for babies born at 25 to 27 weeks. Birth usually comes around the 38th week after fertilisation, at 48 to 53 centimetres long, and full control of voluntary movement keeps developing until puberty.
Animals differ in how ready their newborns are. Altricial species arrive helpless and need lots of care; precocial ones are born mobile with open eyes and fur or down. Primates are precocial, except for humans. Litter-bearing mammals lodge each fetus, wrapped in its own placental tissue, along one of two long uteri rather than in a single uterus. A fetal stage can also let a species time births for a favourable season.
Source: Fetus
