Human evolution is being shaped by the number of women giving birth by caesarean section (C-section), a study has found.

Researchers estimate that between 3% and 6% of births worldwide are obstructed because the mother's pelvis is too small for the head or shoulders.

Although the caesarean section is thought to date from ancient times, it is only in the modern era that its use has mushroomed.

Its use stateside increased from less than 5% of births in the 1970s to more than a quarter for most of the last 30 years, according to the US National Library of Medicine.

It is believed that the size of women's pelvises and the size of babies' heads were finely balanced over millions of years of human evolution, with hips unable to dip below a certain width if babies were to survive.

But the increased use of caesareans means all that is changing because women with hips of any size can now give birth to babies of any size.

The study, by a team from Austria and the US, used a "mathematical model" to show that increased use of caesareans had accounted for an increase in the size of babies relative to women's pelvic canals.

They call this "fetopelvic disproportion".

Bigger babies are now more likely to survive and the number of babies dying in childbirth has dropped significantly.

The paper written by the team, led by Philipp Mitteroecker, said: "Using a simple mathematical model, we show that weak directional selection for a large neonate, a narrow pelvic canal, or both is sufficient to account for the considerable incidence of fetopelvic disproportion.

"Based on this model, we predict that the regular use of caesarean sections throughout the last decades has led to an evolutionary increase of fetopelvic disproportion rates by 10 to 20%."

However the team admits there may be other factors at work.

For example, a change in diet over the generations could also be partly responsible for the mismatch between unborn baby size and pelvis width.