A woman's chance of successful pregnancy drops considerably after age 35, regardless of her reproductive history, according to new research.
More than one in five pregnancies in "older" women end in spontaneous abortion, ectopic pregnancy or stillbirth, researchers report in today's British Medical Journal.
By age 42, more than half of intended pregnancies are unsuccessful, according to the team led by Anne-Marie Nybo Andersen of the Danish Epidemiology Science Centre in Copenhagen.
The knowledge that having a baby becomes more difficult with age is not new. But the finding that these odds do not vary significantly even if a woman has had children, an abortion, or a history of menstrual or fertility problems adds to the knowledge about reproduction.
The researchers analyzed the reproductive histories and outcomes of more than 600,000 Danish women who became pregnant between 1978 and 1992.
The risk of a spontaneous abortion varied from 8 per cent at age 22 to more than 84 per cent by age 48, irrespective of previous reproductive problems.
Similarly, the risk of an ectopic pregnancy also increased with age, from 1.4 per cent of all pregnancies at age 21, to 6.9 per cent by age 44.
An ectopic pregnancy occurs when the fetus remains within the Fallopian tube rather than implanting itself in the lining of the uterus.
The risk of stillbirth was higher among women over the age of 35, but only marginally.
Dr. Andersen and her team suggest that delaying pregnancy not only increases risks for women but adds to health-care costs.
But in an editorial accompanying the research paper, Zena Stein, professor emeritus at the school of public health at Columbia University in New York, argues that the biological disadvantage of older mothers is largely offset by the social advantages afforded her children.
Older parents, she writes, tend to have more experience and knowledge than younger parents and their economic situation is usually better.
She also argues that record-keeping has to be improved. There is insufficient information on how delaying pregnancy affects other aspects of reproduction, such as multiple births and congenital malformations, she writes.