A woman’s risk of having a miscarriage is strongly associated with her age, is considerably higher after previous miscarriages and is also increased after certain pregnancy complications, research from Norway has shown. The authors of the study*, published today in the BMJ, said more focused studies of these associations might lead to new insights regarding the shared underlying causes of pregnancy complications and miscarriage.
The team, led from the Norwegian Institute of Public Health, analysed data from the Medical Birth Register of Norway, the Norwegian Patient Register, and the induced abortion register to estimate the burden of miscarriage in the Norwegian population and to evaluate the associations with maternal age and pregnancy history. These covered all Norwegian women who were pregnant between 2009-13 – a total of 421,201 pregnancies during the study period. They identified miscarriage as pregnancy loss between six and 20 weeks, or fetal death before 20 weeks with a birthweight of less than 500g; a fetal death at 20 gestational weeks or later, or with a birthweight of 500g or more, which was considered a stillbirth.
They found that miscarriage risk was lowest in 25-29 year old women, with the absolute lowest risk at the age of 27 (9.5%), and rose rapidly after the age of 30, reaching 53% in women aged at least 45; mothers younger than 20 had a risk of 15.8%.
There was a strong recurrence risk of miscarriage, with age-adjusted odds ratios (OR) of 1.54 after one miscarriage, 2.21 after two, and 3.97 after three consecutive miscarriages. The risk of miscarriage was modestly but significantly increased if the previous pregnancy had ended in a (live) preterm delivery (OR 1.22), stillbirth (1.30), caesarean section (1.16), or if the woman had gestational diabetes in the previous pregnancy (1.19). The risk of miscarriage was also significantly, though only slightly, higher in women who themselves had been small for gestational age (1.08).
The study authors said the associations of risk of miscarriage with complications in previous pregnancies point to the presence of causal factors that increase the risk of both. They commented: “We found evidence that certain other pregnancy outcomes cluster with the risk of miscarriage, suggesting that these outcomes might share underlying causes. Specifically, the risk of miscarriage was moderately increased among women who had experienced a stillbirth, preterm delivery, or gestational diabetes in their previous pregnancy.”
They pointed out their study could not establish causation as it was observational, and other limitations include possibly missing some early miscarriages where the women did not contact specialist healthcare services – but its strengths included its population-based design, prospective data collection, and the availability of information from the woman’s own birth record.
They concluded: “The risk of miscarriage varies greatly with maternal age, shows a strong pattern of recurrence, and is also increased after some adverse pregnancy outcomes. Miscarriage and other pregnancy complications might share underlying causes, which could be biological conditions or unmeasured common risk factors.”
*Magnus MC, Wilcox AJ, Morken N-H, et al. Role of maternal age and pregnancy history in risk of miscarriage: prospective register based study. BMJ 2019; 364: l869 http://dx.doi.org/10.1136/bmj.l86