Placenta said to key the pregnency lengh

The placenta may play a key role in determining the gestation period of humans and other mammals, according to a new study.

The research links growth rates of mammals inside the womb to the structure of the placenta and the way it connects mother and baby.
This, as well as mammal size, may explain variations in gestation times.
While a human pregnancy is nine months long, a meadow mouse gives birth after just three weeks.
Finger-like
The placenta plays a fundamental role in reproduction in mammals, transferring nutrients and oxygen from mother to foetus and removing waste from the foetus.
The structure of the placenta, however, varies enormously from species to species. This new study suggests these variations may play a role in the length of the pregnancy.
The researchers analysed 109 mammal species. What they found was the more complex and folded the placenta of a mammal, the shorter the gestation time appeared to be.
What they believe is happening is that more folded placentas are able to deliver more nutrients to the infant, spurring growth, and so reducing gestation times.
More complex placenta were found in mice, which have a gestation period of some three weeks, dogs (two months) and leopards (three months).
But humans and other primates such as baboons were seen to have simpler placentas where fewer nutrients were passing into the foetus, resulting in slower growth rates and longer gestation times.
"In humans, the placenta has simple finger-like branches with a relatively limited connection between the mother's tissues and those of the foetus," said Dr Isabella Capellini of Durham University, the lead author on the study.
"Whereas in leopards, the placenta forms a complex web of interconnections that create a larger surface area for the exchange of nutrients."
The study is published in the journal American Naturalist.