Evidence summary
Early evidence suggested benefit from IM progesterone
A 2003 RCT compared weekly IM progesterone (n = 310) and placebo (n = 153) injections in women with a history of spontaneous preterm delivery. Participants were at 15w0d to 20w3d of a singleton pregnancy with no fetal abnormality. The 17-OHP group, compared to the placebo group, had significantly fewer deliveries at < 37 weeks (36.3% vs 54.9%; relative risk [RR] = 0.66; 95% CI, 0.54 to 0.81; number needed to treat [NNT] = 6), at < 35 weeks (20.6% vs 30.7%; RR = 0.67; 95% CI, 0.48 to 0.93; NNT = 10), and at < 32 weeks (11.4% vs 19.6%; RR = 0.58; 95% CI, 0.37 to 0.91; NNT = 13).1 There were significantly lower rates of necrotizing enterocolitis, intraventricular hemorrhage, and need for supplemental oxygen in infants of women in the treatment group.1 The study was underpowered to detect neonatal morbidity.
A 2013 Cochrane Review (5 studies including the 2003 RCT; 602 women) found that 17-OHP led to a decreased risk of birth at < 34 weeks (RR = 0.31; 95% CI, 0.14-0.69). It also led to a significant reduction in perinatal and neonatal mortality, birth at < 37 weeks, birthweight < 2500 g, use of assisted ventilation, incidence of necrotizing enterocolitis, and admission to the neonatal ICU.2
In a large follow-up study, progesterone did not demonstrate benefit
The PROLONG study was a double-blind, placebo-controlled international RCT of women with a previous singleton spontaneous preterm birth. The study involved 93 clinical centers in 9 countries: 41 in the United States and 52 outside the United States. The PROLONG study was much larger than the 2003 study: 1139 active treatment (vs 310) and 578 placebo (vs 153) participants. Women were randomized 2:1 to receive either 250 mg 17-OHP or inert oil placebo weekly from 16w0d-20w6d until 36 weeks. The outcome measures were: (1) delivery at < 35 weeks and (2) a neonatal morbidity composite index. This composite index included any of the following: neonatal death, grade 3 or 4 intraventricular hemorrhage, respiratory distress syndrome, bronchopulmonary dysplasia, necrotizing enterocolitis, and proven sepsis.3
Progesterone did not improve any of the studied outcomes: there were no significant differences in the frequency of birth at < 35 weeks (17-OHP 11% vs placebo 11.5%; RR = 0.95; 95% CI, 0.71-1.26), in neonatal morbidity index (17-OHP 5.6% vs placebo 5%; RR = 1.12; 95% CI, 0.68-1.61), and in frequency of fetal/early infant death (17-OHP 1.7% vs placebo 1.9%; RR = 0.87; 95% CI, 0.4-1.81).3 In the United States subgroup (n = 391; 23% of all patients), there was no significant difference in rate of birth at < 35 weeks (17-OHP 15.6% vs placebo 17.6%; RR = 0.88; 95% CI, 0.55-1.40).3
However, PROLONG had some limitations. Importantly, the 2003 RCT included 183 (59%) non-Hispanic Black women in the experimental group and 90 (58.5%) in the control group, whereas the 2020 PROLONG study had only 6.6% non-Hispanic Black participants. The neonatal outcome data for the PROLONG study only included 6 Black women in the experimental arm and 3 in the control arm.3,4 Black women have prematurity rates that are 2 to 3 times higher than those in White women.5
Additionally, the PROLONG study had fewer smokers and more women who were married/living with a partner. Compared with prior studies, the PROLONG study had a lower proportion of women with > 1 spontaneous preterm birth and fewer with a shortened cervix (< 2%).3 As a result of having lower risk participants, PROLONG may have been underpowered to detect improvements in outcome.3
A subsequent meta-analysis suggests some benefit for high-risk women
The 2021 Evaluating Progestogens for Preventing Preterm birth International Collaborative (EPPPIC) meta-analysis of individual data from 31 RCTs—involving 11,644 women and 16,185 babies—found that, compared with placebo, 17-OHP for women with a history of preterm delivery or short cervix did not significantly decrease the number of babies born before 34 weeks (5 trials [including the 2003 RCT and PROLONG studies]; 3053 women; RR = 0.83; 95% CI, 0.68–1.01).6 However, it found that vaginal progesterone significantly decreased birth prior to 34 weeks (9 trials; 3769 women; RR = 0.78, 95% CI, 0.68-0.90).6 The authors concluded that both IM and vaginal progesterone decreased preterm delivery in high-risk women. The effect was stronger for women with a short cervix than for women with a history of preterm delivery.6
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