One of the largest studies finds that women who have had three or more abortions have a higher risk of some adverse birth outcomes, such as delivering a baby prematurely and with a low birth weight.
The research, which is published online in Europe's leading reproductive medicine journal Human Reproduction  today (Thursday), found that among 300,858 Finnish mothers, 31,083 (10.3%) had had one induced abortion between 1996-2008, 4,417 (1.5%) had two, and 942 (0.3%) had three or more induced abortions before a first birth (excluding twins and triplets). Those who had had three or more induced abortions had a small, but statistically significant increased risk of having a baby with very low birth weight (less than 1500g), low birth weight (less than 2500g), or of a preterm birth (before 37 weeks), or very preterm birth (before 28 weeks), compared to women who had had no abortions. There was a slightly increased risk of a very preterm birth for women who had had two induced abortions.
Dr Reija Klemetti, an associate professor and senior researcher in public health at the National Institute for Health and Welfare in Helsinki, Finland, who led the research, said: "Our results suggest that induced abortions before the first birth, particularly three or more abortions, are associated with a marginally increased risk during the first birth. However, the increased risk is very small, particularly after only one or even two abortions, and women should not be alarmed by our findings."
The risk of having a baby born very preterm appeared to increase slightly with each induced abortion, but only the risk from two abortions or more was statistically significant.
"To put these risks into perspective, for every 1000 women, three who have had no abortion will have a baby born under 28 weeks," said Dr Klemetti. "This rises to four women among those who have had one abortion, six women who have had two abortions, and 11 women who have had three or more."
Among women who had had three or more abortions, there was a statistically significant increased risk of a third (35%) of having a baby born preterm (before 37 weeks), a two-fold (225%) increased risk of very low birth weight, and a two-fifths (43%) increased risk of low birth weight.
The study also showed a small increased risk of a baby's death around the time of birth. However, the numbers for this finding were very low (1498 births or five per 1000 babies) and so should be treated with caution. In addition, the authors say they might not have been able to fully adjust for all the factors that could affect this result and perinatal deaths are sensitive to social factors such as poverty.
"Our study is the first large study to look at a broad set of perinatal outcomes and to control, at least partly, for the most important confounding factors such as smoking and socioeconomic position," said Dr Klemetti. "However, it is important to say that even though we adjusted for these factors, and also ectopic pregnancies and miscarriages, there might be some confounding for social class that we could not control for. Most probably, this may be related to women's (or some of these women's) way of life, life habits, and sexual and reproductive health.
"Furthermore, this is an observational study and, however large and well-controlled, it only shows there is a link between abortion and some adverse birth outcomes - it cannot prove that abortions are the cause.
"Finland has one of the lowest rates of induced abortion in Europe , but even so, a large number are carried out every year. In addition, Finland has good quality abortion and maternity care, and in other contexts, particularly in poorer countries, the situation may be different. For these reasons, even a very small increase in the risk of poor birth outcomes could have significant health implications, as preterm births and low birth weight can have serious, adverse effects on the health and well-being of both babies and mothers.
"We suggest that the potential for increased risks for subsequent births should be included in sex education, especially as there are other, good reasons to avoid induced abortions. Health professionals should also be informed about the potential risks of repeat abortions."