Women who have had an abortion or miscarriage are more likely to give birth to premature or underweight babies, according to research.

A single miscarriage or abortion almost triples the risk of a subsequent baby being born weighing under 2.5kg (5lb 7oz). Low birthweight is strongly linked to deaths in the first week of life or during infancy.

The risks increased sharply for women who had several miscarriages or abortions, although the study did not discriminate between the two. Previously, researchers have failed to find a consistent pattern between abortion and risks to future pregnancies. Some studies showed that the risks increased while others did not.

A team of public health doctors from Virginia Commonwealth University, in the US, determined to settle the question by using the details of more than 45,000 births.

The birth records were linked to details of the mother’s medical history, but did not distinguish between abortion and miscarriage. They related to 1959-66, when abortion was far less common and women would have been reluctant to admit to having had one.

The results, published in Journal of Epidemiology and Community Health, show a single abortion or miscarriage almost tripled the risk of low birthweight; two increased the risk about fivefold and three increased the risk ninefold. A miscarriage or abortion increased the risk of a subsequent baby being premature by 70 per cent, two doubled the risk and three tripled it.

The results hold true even after adjusting for other factors, such as smoking, high blood pressure and heavy drinking.

The pattern of results may give hints about the most probable cause. Two mechanisms have been proposed: a failure of the cervix (the lower part of the womb) as a result of damage done by the ending of the previous pregnancy, or internal damage to the cervix and womb as a result of complications, such as infections.

The first appears to be ruled out because low birthweight was found both for babies born early and for those that went to term. If cervical incompetence was the cause, a stronger association would be expected for babies that had been delivered early.

So the likeliest explanation, the authors suggest, is that infections or adhesions in the womb slow the growth of subsequent babies.

The authors, led by Tilahun Adera, say that though the study is the largest done in the US, it has limits, the most salient being that the data did not distinguish between abortion and miscarriage. If women had had an abortion but hidden it abortions would be underreported and links with abortion and miscarriage weakened.

A spokesman for the Royal College of Obstetricians and Gynaecologists said that the study was carried out with a group that did not reflect the current UK population. He added that modern medical techniques reduced the risk of women who had had an abortion subsequently giving birth to premature or underweight babies.

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