Risk of miscarriage increases with age, after consecutive previous miscarriages
The risk of miscarriage is lowest at age 27 years but increases with older age and by up to four times after three consecutive previous miscarriages, according to a Norway study. This risk is also associated with earlier pregnancy complications, including stillbirth, preterm delivery and gestational diabetes.
“Miscarriage and other pregnancy complications might share underlying causes, which could be biological conditions or unmeasured common risk factors,” researchers said.
This prospective register-based study obtained data from the Medical Birth Register of Norway, the Norwegian Patient Register and the induced abortion register. Participants included all Norwegian women who were pregnant between 2009–2013. Logistic regression was used to estimate the risk of miscarriage according to the woman’s age and pregnancy history.
A total of 421,201 pregnancies were included in the analysis during the study period. The lowest risk of miscarriage (10 percent) was found in women aged 25–29 years, but this risk increased rapidly after age 30 years, which reached 53 percent in those aged ≥45 years. [BMJ 2019;364:l869]
Risk of recurrence was highly likely after a previous miscarriage, with age-adjusted odds ratios (ORs) of 1.54 (95 percent CI, 1.48–1.60) after one miscarriage, 2.21 (2.03–2.41) after two and 3.97 (3.29–4.78) after three consecutive miscarriages.
“Women vary in their risk of miscarriage at a given age, for reasons that are not well understood. A miscarriage marks a woman as being at relatively higher risk, and this risk is expressed in subsequent pregnancies,” researchers said.
A modest increase in the risk of miscarriage was observed if the previous birth ended in a preterm delivery (adjusted OR, 1.22; 1.12–1.29), stillbirth (adjusted OR, 1.30; 1.11–1.53), caesarean section (adjusted OR, 1.16; 1.12_1.21) or if the woman had gestational diabetes in the previous pregnancy (adjusted OR, 1.19; 1.05–1.36). Additionally, miscarriage risk was slightly higher in women who themselves were born small for gestational age (adjusted OR, 1.08; 1.04–1.13).
“Our results for preterm delivery are supported by the temporally reverse association, with previous studies reporting a higher risk of preterm birth among women with a history of miscarriage,” researchers said. [Hum Reprod 2015;30:1239-1245; BJOG 2015;122:1525-1534; Am J Obstet Gynecol 2004;191:1225-1231; Eur J Obstet Gynecol Reprod Biol 2011;159:342-346; Acta Obstet Gynecol Scand 2016;95:1383-1390; Am J Obstet Gynecol 2011;205:286.e1-5]
Cervical insufficiency and infections are possible shared pathways, but these are speculative, according to researchers. Previous studies suggest that glucose metabolism abnormalities could increase both the risk of miscarriage and preterm delivery. [Am J Perinatol 2016;33:1300-1305; Diabet Med 2014;31:357-365; BMC Res Notes 2012;5:496; Gynecol Obstet Invest 2011;72:245-251]
“We also observed a small increase in the risk of miscarriage after a caesarean section. A systematic review of caesarean section and subsequent risk of miscarriage has reported inconsistent effects, with relative risk or OR estimates ranging from 0.76–1.32,” researchers said. [PLoS One 2013;8:e54588]
“More focused studies of these associations might lead to new insights regarding the shared underlying causes of pregnancy complications and miscarriage,” they noted.