Why nine months? The answer’s in your placenta

Scientists discover why it takes so long for us mums to grow our babies


The placenta is the key to how long we and other mammals are pregnant for, according to new research. Nine months is needed for us to grow a baby because the shape of a human placenta only lets a limited amount of nutrients through at a time.


The more folded the placenta, the more connections between mum and child. This means that animals with more complex placentas, such as mice, dogs and leopards, all have relatively short pregnancies – three weeks, two months and three months respectively.

“In humans, the placenta has simple finger-like branches with a relatively limited connection between the mums’s tissues and those of the foetus,” said the lead author of the study, Dr Isabella Capellini.  “Whereas in leopards, the placenta forms a complex web of interconnections that create a larger surface area for the exchange of nutrients.”

There are other factors such as the size of the animal, but this battle for nutrients between mum and baby is universal in mammals. The research suggests that while mice babies are winning the nutrient battle and pop out after just three weeks, human mums and babies are more evenly matched which makes the process slower.


If you’re nearing the finishing line, spare a thought for all those elephant mums-to-be out there who have a further year and a month to go!

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