We know it’s the unborn baby that triggers birth when it’s ready by sending signals to the womb to get things going.

This is just the final act of growing a baby. It’s been bossing the placenta and womb for nine months, keeping the food supply coming, so it can grow and blood vessels can expand so ­nutrients can be piped from the mum via the placenta.

Now a new study has learned that a key signal from the unborn baby keeps the growth balance just right and could help explain why some babies grow poorly in the womb.

As a foetus grows it needs to update its mother continuously about its increasing needs for food.

It receives its nourishment via blood vessels in the placenta, a specialised organ that’s the most intimate interface between baby and mother.

Dr Ionel Sandovici, the Cambridge University study’s first author, says: “As it grows in the womb, the foetus needs food from its mum, and healthy blood vessels in the placenta are essential to help it get the correct amount of nutrients it needs.

“We’ve identified one way that the foetus uses to communicate with the placenta to prompt the correct expansion of these blood vessels. When this communication breaks down, the blood vessels don’t develop properly and the baby will struggle to get all the food it needs.”

Between 10% and 17% of babies grow poorly in the womb, often with scantily developed blood vessels in the placenta which, incidentally, between mid- and late pregnancy should reach a total length of about 320 kilometres!

The study team used mice to demonstrate how the foetus sends a signal to encourage growth of blood vessels within the placenta and command more ­nutrients from the mother.

That signal is known as IGF2. Too much IGF2 and there’s too much growth, too little and growth is impeded. Both are bad.

Babies that are too large or too small are more likely to suffer or even die at birth, and have a higher risk of developing diabetes and heart problems as adults. So managing IGF2 in the womb could head off these future health hazards.

The team says their findings will give a better understanding of how the foetus, placenta and mother communicate during pregnancy.

This in turn could lead to ways of measuring IGF2 levels in the foetus and finding out how to use medication to normalise these levels, or promote normal development of placental blood vessels.