According to a group of London-based fertility specialists the increase in women over 35 having children is cause for some alarm.

They say the best age for pregnancy remains 20 to 35.

The specialists, led by Dr Susan Bewley, who treats women with high-risk pregnancies at Guy's and St Thomas' Hospital, warn age-related fertility problems increase after 35 and dramatically after 40.

In the British Medical Journal, the specialists write, "Paradoxically, the availability of IVF [in-vitro fertilization] may lull women into infertility while they wait for a suitable partner and concentrate on their careers and achieving security and a comfortable living standard."

Once an older woman does become pregnant, she runs a greater risk of miscarriage, fetal and chromosomal abnormalities, and pregnancy-related diseases.

They add, "Women want to 'have it all' but biology is unchanged; deferring defies nature and risks heartbreak."