showing that the likelihood of having a baby more than three weeks early among women aged over 40 was 20% higher than those in their early 30s. However, the actual risk remained low, increasing from 1% to 1.2%.

There was also a slightly higher risk for women under 30, thought to be related to higher rates of drug use and smoking, and the fact mothers were likely to be in their first pregnancy.

The study, published in the journal Plos One, also confirmed previously identified direct links between rising age and increasing risks of pregnancy-related diabetes, high blood pressure, and other complications.

Globally there has been a steady increase in women over the age of 40 giving birth.

For the study, researchers in Canada and France took the records of 180,000 pregnant Canadian women. After removing categories such as women carrying more than one baby, they analyzed more than 165,000 remaining cases.

The results were then adjusted for previous medical problems, pregnancy-related diabetes and the higher likelihood that older mothers had used IVF and other medical help to get pregnant.

The researchers acknowledged other studies had found similar “u-curves”, where the risk of premature birth declined and then rose again sharply for older mothers, but they pointed out that their paper was based on a huge number of cases, covering different situations such as variations between urban and rural mothers, for example.

There is a concern, however, that studies into the risks of being an older mother distort the risk. Mothers over 40 have also pointed out that the risks of having children, later on, are well known, but seldom deter a woman who wants a baby.