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Studies suggest decreased efficacy of 17 α-hydroxyprogesterone caproate (17OHPC) in patients with a BMI ≥25 in preventing recurrent preterm birth (rPTB). This may be related to differential effects of 17OHPC on progesterone (P) and estriol (E3) levels, which, when elevated, have been associated with decreased and increased rates of PTBs, respectively. Our objective was to evaluate the incidence of rPTB by BMI and 17OHPC exposure, and to assess whether this incidence is explained by the changes in P or E3 levels during pregnancy.
This was a secondary analysis of a randomized clinical trial of 17OHPC to prevent rPTB. In the parent study, baseline and longitudinal salivary P and E3 samples were collected. 80 were randomly selected for a prior planned case-control study comparing P and E3 concentrations by 4 groups of 20 each: +PTB/17OHPC, +PTB/placebo, -PTB/17OHPC and -PTB/placebo. Our current study analyzed the incidence of rPTB for women in the parent trial with available BMI and outcome data, stratifying BMI by <25 and ≥25, and by 17OHPC exposure. Multivariable logistic regression was performed to assess the interaction of BMI and treatment groups with rPTB (<37 weeks), adjusting for confounders. Of 80 patients with longitudinal salivary samples, 75 had data on BMI. Salivary P and E3 levels were measured in 5-week intervals. Longitudinal analysis used mixed models to determine whether an interaction of BMI and 17OHPC impacted the trajectory of salivary P and E3.
17OHPC decreased rPTB compared to placebo for BMI<25, but not BMI≥25 (p=0.017, Table 1). In the group with longitudinal testing, there was no significant difference in the rate of change in salivary P levels between groups. However, in women with a BMI <25, 17OHPC mitigated the increase in E3 levels compared with placebo (p = 0.028, Fig. 1).
Women treated with 17OHPC with BMI<25 have a blunted increase in salivary E3 compared to those with BMI≥25, possibly contributing to lower rPTB rates in this group. Dosing studies should be considered for women with BMI ≥25.