Role of maternal age and pregnancy history in risk of miscarriage: prospective register based study

Objectives: To estimate the burden of miscarriage in the Norwegian population and to evaluate associations with maternal age and pregnancy history.
Design: Prospective cohort study.
Setting: All registered pregnancies in Norway.
Participants: All Norwegian women who were pregnant between 2009 and 2013, as identified through the Medical Birth Register of Norway, the Norwegian Patient Register and the abortion register.
Main outcome measures: Risk of miscarriage according to the woman’s age and pregnancy history using logistic regression.
Results: There were 421,201 pregnancies during the study period. Miscarriage risk was lowest among women age 25-29 years (10%), and rose rapidly after age thirty, reaching 53% among women 45 years and older. There was a strong recurrence risk of miscarriage, with age-adjusted odds ratios (aORs) of 1.5 (95% CI: 1.5 to 1.6) after one miscarriage, 2.2 (95% CI: 2.0 to 2.4) after two, and 4.0 (95% CI: 3.3 to 4.8) after three consecutive miscarriages. Risk was modestly increased if the previous birth ended in a preterm delivery (1.22; 95% CI: 1.12 to 1.29), stillbirth (1.30; 1.11 to 1.54), caesarean section (1.16 95% CI: 1.12 to 1.21), or if woman had gestational diabetes in the previous pregnancy (1.19; 95% CI: 1.05 to 1.36). Risk of miscarriage was slightly higher among women who themselves had been small-for-gestational age (1.08; 95% CI: 1.04 to 1.13).
Conclusions: Risk of miscarriage varies greatly with maternal age, shows a strong pattern of recurrence, and risk is also increased after some adverse pregnancy outcomes. Miscarriage and other pregnancy complications may share underlying causes, which could be biological conditions or unmeasured common risk factors.