Women who delay having children enter a ‘risk zone’ of problems as early as age 30 according to new research.
The risk of problems such as premature and stillbirth rises by as much as 20% for women aged between 30 and 34, compared with those having babies in their late twenties.
First-time mothers have previously been told they are at high risk above the age of 35, but an increasing number of women are putting off having children until later.
In England and Wales the average age at first birth was 27.9 years in 2011 – up 1.3 years in a decade – and the average age of all mothers was almost 30.
Study lead Professor Ulla Waldenström, from the Karolinka Institute in Sweden, said: "To our surprise we found an absolute increase in risk for negative effects on pregnancy outcomes in the age group 30-34.
"These are independent of the effects of smoking and being overweight, which, when combined, lead to an even greater risk."
Her team compared the results of pregnancy in first-time mothers over the age of 30 with those aged 25 to 29.
The results showed that for women in their early thirties, there was a one-fifth higher risk of giving birth very prematurely – during weeks 22 to 31 of pregnancy – or having a stillbirth. There was also a higher risk that the unborn baby’s growth would be restricted and that the baby would die.
First-time mothers in this age group were not previously seen as at risk.
Smoking and being overweight pushed up the risks of pre-term birth, stillbirth and neonatal mortality to the same level as that for women aged 35-39.
Professor Waldenström concluded: "For women individually, the risk is small, but for society at large there will be a significant number of 'unnecessary' complications with so many women having children just after 30.
"It would therefore be advisable to inform both women and men, even at schools, of how important age is to child birth. Biologically the best time is probably 20 to 30."