Women who put off getting pregnant till past their mid-thirties are defying nature and risk the heartbreak of infertility, miscarriage or other complications, doctors warned on Friday.

"Women want to 'have it all,' but biology is unchanged," they said in an editorial in the British Medical Journal.

An increasing number of women in Western countries are delaying having a baby till later, many for career reasons.

But women over 35 face a greater chance of suffering age-related fertility problems including miscarriages, ectopic pregnancies, twin births, bleeding and high blood pressure.

Once women pass 40 these risks increase dramatically, said consultant obstetrician Susan Bewley, one of the editorial's authors.

"We are seeing the disease burden of this social trend going up," she said.

"As the number of older mothers is going up, most are fine, but in our gynecological and obstetric clinics we are seeing a lot of casualties of this trend," she told BBC radio.

Bewley and her colleagues warned in their editorial public health authorities were ignoring an "epidemic of pregnancy in middle age."

"Doctors and healthcare planners need to grasp this threat to public health and support women to achieve biologically optimal childbearing," they wrote.