Spontaneous preterm birth remains a leading cause of neonatal morbidity and mortality among nonanomalous neonates in the United States. Spontaneous preterm birth tends to recur at similar gestational ages. Intramuscular 17-alpha hydroxyprogesterone caproate reduces the risk of recurrent spontaneous preterm birth. Unfortunately, one-third of high-risk women will have a recurrent spontaneous preterm birth despite 17-alpha hydroxyprogesterone caproate therapy; the reasons for this variability in response are unknown.

We hypothesized that clinical factors among women treated with 17-alpha hydroxyprogesterone caproate who suffer recurrent spontaneous preterm birth at a similar gestational age differ from women who deliver later, and that these associations could be used to generate a clinical scoring system to predict 17-alpha hydroxyprogesterone caproate response.

Secondary analysis of a prospective, multicenter, randomized controlled trial enrolling women with ≥1 previous singleton spontaneous preterm birth <37 weeks’ gestation. Participants received daily omega-3 supplementation or placebo for the prevention of recurrent preterm birth; all were provided 17-alpha hydroxyprogesterone caproate. Women were classified as a 17-alpha hydroxyprogesterone caproate responder or nonresponder by calculating the difference in delivery gestational age between the 17-alpha hydroxyprogesterone caproate–treated pregnancy and her earliest previous spontaneous preterm birth. Responders were women with pregnancy extending ≥3 weeks later compared with the delivery gestational age of their earliest previous preterm birth; nonresponders delivered earlier or within 3 weeks of the gestational age of their earliest previous preterm birth. A risk score for nonresponse to 17-alpha hydroxyprogesterone caproate was generated from regression models via the use of clinical predictors and was validated in an independent population. Data were analyzed with multivariable logistic regression.

A total of 754 women met inclusion criteria; 159 (21%) were nonresponders. Responders delivered later on average (37.7±2.5 weeks) than nonresponders (31.5±5.3 weeks), P<.001. Among responders, 27% had a recurrent spontaneous preterm birth (vs 100% of nonresponders). Demographic characteristics were similar between responders and nonresponders. In a multivariable logistic regression model, independent risk factors for nonresponse to 17-alpha hydroxyprogesterone caproate were each additional week of gestation of the earliest previous preterm birth (odds ratio, 1.23; 95% confidence interval, 1.17−1.30, P<.001), placental abruption or significant vaginal bleeding (odds ratio, 5.60; 95% confidence interval, 2.46−12.71, P<.001), gonorrhea and/or chlamydia in the current pregnancy (odds ratio, 3.59; 95% confidence interval, 1.36−9.48, P=.010), carriage of a male fetus (odds ratio, 1.51; 95% confidence interval, 1.02−2.24, P=.040), and a penultimate preterm birth (odds ratio, 2.10; 95% confidence interval, 1.03−4.25, P=.041). These clinical factors were used to generate a risk score for nonresponse to 17-alpha hydroxyprogesterone caproate as follows: black +1, male fetus +1, penultimate preterm birth +2, gonorrhea/chlamydia +4, placental abruption +5, earliest previous preterm birth was 32−36 weeks +5. A total risk score >6 was 78% sensitive and 60% specific for predicting nonresponse to 17-alpha hydroxyprogesterone caproate (area under the curve=0.69). This scoring system was validated in an independent population of 287 women; in the validation set, a total risk score >6 performed similarly with a 65% sensitivity, 67% specificity and area under the curve of 0.66.

Several clinical characteristics define women at risk for recurrent preterm birth at a similar gestational age despite 17-alpha hydroxyprogesterone caproate therapy and can be used to generate a clinical risk predictor score. These data should be refined and confirmed in other cohorts, and women at high risk for nonresponse should be targets for novel therapeutic intervention studies.