Delaying having children defies nature and risks heartbreak, say experts in this week's BMJ. If women want families and room for manoeuvre they are unwise to wait till their 30s.

They believe the best age for childbearing remains 20-35 and call on doctors and healthcare planners to support women to achieve "biologically optimal childbearing."

Pregnancies in women older than 35 are increasing markedly in Western countries, write Susan Bewley and colleagues. Age related fertility problems increase after 35 and dramatically after 40, and once a woman is pregnant, outcomes for both the mother and child are poorer.

Delaying also affects partners, as semen counts deteriorate gradually every year, and children of older men have an increased risk of schizophrenia and several genetic disorders.

Despite these difficulties, most pregnancies in women older than 35 have good outcomes, but obstetricians and gynaecologists have seen dramatic demographic changes in two decades and are witnesses to the resultant tragedies, they write.

It is ironic that as society becomes more risk averse and pregnant women more anxious than in the past, a major preventable cause of this ill health and unhappiness is unrecognised, they say. "Public health agencies target teenagers but ignore the epidemic of pregnancy in middle age."

Women want to "have it all" but biology is unchanged. Their delays may reflect disincentives to earlier pregnancy or maybe an underlying resistance to childbearing as, despite the advantages brought about by feminism and equal opportunities legislation, women still bear full domestic burdens as well as work and financial responsibilities.

The reasons for these difficulties lie not with women but with a distorted and uninformed view from society, employers, and health planners. Doctors and healthcare planners need to grasp this threat to public health and support women to achieve biologically optimal childbirth, they conclude.