The risk of miscarriage increases with maternal age, has a strong recurrence risk, and is increased after some pregnancy complications, a new study published in BMJ shows.
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These findings indicate that there may be common risk factors for both pregnancy complications and miscarriage, says researcher Maria C. Magnus at the Center for Fertility and Health, and first author of the study.
Spontaneous miscarriage is thought to be quite common, but it is not easy to study the risk in most countries due to inadequate registration. Norway is one of the few countries where it is possible to study miscarriages at the population level based on national health registers.
A group of researchers at the Center for Fertility and Health at the Norwegian Institute of Public Health used the national health registers to estimate the risk of miscarriage among Norwegian women and to assess the connection with age and pregnancy history.
They identified all pregnancies in Norway between 2009 and 2013 using the Medical Birth Registry of Norway, the Norwegian Patient Registry and the Norwegian Registry of Pregnancy Termination.
This made it possible to study the risk of miscarriage related to age and previous pregnancy complications from all registered pregnancies in Norway.
There were a total of 421,201 pregnancies during the study period.
The risk of a pregnancy ending in miscarriage was 12.8%. The proportion of pregnancies that ended in miscarriage was lowest among women between 25-29 years (10%), increased sharply after 30 years, and 53% of pregnancies among women who were 45 or older ended with miscarriage.
There was also a strong repeat risk for miscarriage. Compared to women pregnant for the first time, the risk of new spontaneous abortion increased by 50% after a miscarriage, and there was twice as much risk of spontaneous abortion if the woman had two previous spontaneous abortions, and there were four times higher risk among them with three previous abortions.
Earlier pregnancy complications also gave a higher risk of miscarriage. Women who had experienced a premature birth, delivery by caesarean section, or gestational diabetes in the previous pregnancy had an increased risk of miscarriage in the next.
In addition, women who were classified with poor fetal growth when she was born herself had a somewhat increased risk of miscarriage as an adult.
– This is a descriptive study and we can therefore conclude about causal relationships, says Magnus. The study also did not capture spontaneous abortions that did not result in contact with the specialist health service.
– We hope that further research into underlying causes of problems in pregnancy can help us explain the relationship between pregnancy complications and miscarriage, says Magnus. ■