EVIDENCE SUMMARY
To evaluate the longstanding belief that a short IPI after miscarriage is associated with adverse outcomes in subsequent pregnancies, a 2017 systematic review and meta-analysis of 16 studies (3 randomized controlled trials [RCTs] and 13 retrospective cohort studies) with a total of more than 1 million patients compared IPIs shorter and longer than 6 months (miscarriage was defined as any pregnancy loss before 24 weeks).1 The meta-analysis included 10 of the studies (2 RCTs and 8 cohort studies), with a total of 977,972 women and excluded 6 studies because of insufficient data. The outcomes investigated were recurrent miscarriage, preterm birth, stillbirth, pre-eclampsia, and low birthweight in the pregnancy following miscarriage.
Only 1 study reported the specific gestational age of the index miscarriage at 8.6 ± 2.8 weeks.2 All studies adjusted data for age, and some considered other confounders, such as race, smoking status, and body mass index (BMI).
Women included in the meta-analysis were from Asia, Europe, South America, and the United States and had a history of at least 1 miscarriage.1 A study of 257,908 subjects (Conde-Agudelo) also included women with a history of induced abortion from Latin American countries, where abortion is illegal, and made no distinction between spontaneous and induced abortions in those data sets.3 Women with a history of illegal abortion could be at greater risk of subsequent miscarriage than women who underwent a legally performed abortion.
IPI shorter than 6 months carries fewer risks
Excluding the Conde-Agudelo study, women with an IPI < 6 months, compared with > 6 months, had lower risks of subsequent miscarriage (7 studies, 46,313 women; risk ratio [RR] = 0.82; 95% confidence interval [CI], 0.78-0.86) and preterm delivery (7 studies, 60,772 women; RR = 0.79; 95% CI, 0.75-0.83); a higher rate of live births (4 studies, 44,586 women; RR = 1.06; 95% CI, 1.01-1.11); and no increase in stillbirths (4 studies, 44,586 women; RR = 0.88; 95% CI, 0.76-1.02), low birthweight (4 studies, 284,222 women; RR = 1.05; 95% CI, 0.48-2.29) or pre-eclampsia (5 studies, 284,899 women; RR = 0.95; 95% CI, 0.88-1.02) in the subsequent pregnancy.
Including the Conde-Agudelo study, the risk of preterm delivery was the same in women with an IPI < 6 months and > 6 months (8 studies, 318,880 women; RR = 0.93; 95% CI, 0.58-1.48).1 Four of the 10 studies evaluated the risk of miscarriage with an IPI < 3 months compared with > 3 months and found either no difference or a lower risk of subsequent miscarriage.2,4-6
IPI shorter than 3 months has lowest risk of all
A 2017 prospective cohort study examined the association between IPI length and risk of recurrent miscarriage in 514 women who had experienced recent miscarriage (defined as spontaneous pregnancy loss before 20 weeks of gestation).7 Average gestational age at the time of initial miscarriage wasn’t reported. Study participants were 30 years of age on average and predominantly white (76.8%); 12.3% were black.
The authors compared IPIs of < 3 months, 3 to 6 months, and > 18 months with IPIs of 6 to 18 months, which correlates with the IPIs recommended by the World Health Organization (WHO).8 They adjusted for maternal age, race, parity, BMI, and education. An IPI < 3 months was associated with the lowest risk of subsequent miscarriage (7.3% compared with 22.1%; adjusted hazard ratio = 0.33; 95% CI, 0.16-0.71). Women with IPIs of 3 to 6 months and > 18 months didn’t experience statistically significant differences in subsequent miscarriage rates compared with IPIs of 6 to 18 months.7
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