Association of Body Mass Index with Time to Live Birth and Pregnancy in Subfertile Couples Undergoing Restorative Reproductive Medicine

Delayed time to live birth can cause emotional strain, relationship stress, and disrupt family planning. Previous findings on the association between body mass index (BMI) and fecundity in those trying to conceive without medical assistance and those undergoing assisted reproductive technology have been mixed, while females undergoing restorative reproductive medicine (RRM) with normal BMI are more likely to achieve a live birth. However, only one study to date has investigated whether BMI influences fecundity among individuals and couples who received RRM. We examined the relationship between BMI in couples who received RRM, individually and combined, with time to live birth and pregnancy.

Data from the International NaProTechnology Evaluation and Surveillance of Treatment (iNEST) study were analyzed, with additional clinical information manually abstracted from electronic health records. The cohort included couples who pursued natural procreative technology treatment at participating Utah clinical sites who participated in iNEST. BMI was calculated from clinical measurements of height and weight at the start of treatment. Baseline characteristics were summarized using descriptive statistics. Time to live birth and pregnancy were evaluated using Cox proportional hazards models for BMI categories (dichotomized as <25 versus ≥25 kg/m2 and further classified as <18.5, 18.5-24.9, 25-29.9, 30-34.9, and ≥35 kg/m2) for females, males, and couples combined, adjusting for age, parity, and smoking status at start of treatment. When proportional hazards assumptions were violated, accelerated failure time models were fitted. Hazard ratios (HRs) were visualized using forest plots.

Preliminary analysis included 105 couples. Median age was 31.9 years (interquartile range [IQR]: 27.8-36.1) for females and 32.8 years (IQR: 29.5-36.3) for males. Median BMI was 23.1 kg/m2 (IQR: 20.7-28.2) for females and 26.7 kg/m2 (IQR: 24.7-31.0) for males. Among females, 55 (52%) achieved a live birth, and 66 (63%) achieved pregnancy. Additionally, 40 (47%) had at least 1 prior live birth, 97 (92%) reported never smoking, 6 (6%) reported former smoking, and 2 (2%) reported current smoking at their initial RRM clinic visit. In both unadjusted and adjusted Cox proportional hazards models, male BMI ≥ 35 was significantly associated with longer time to live birth (HR: 0.26; adjusted 95% confidence interval [CI]: 0.08, 0.90), and time to pregnancy (HR: 0.26; adjusted 95% CI: 0.08, 0.85) versus BMI 18.5–24.9 kg/m2. Similar trends toward longer times to pregnancy for other higher BMI categories in either partner were observed, but not statistically significant.

In this preliminary analysis of subfertile couples undergoing RRM, male BMI ≥ 35 was associated with significantly longer time to live birth and pregnancy. Female and couple BMI had similar trends for longer time to live birth in both male and female BMI categories, but were not statistically significant. These findings warrant further investigation in larger cohorts to clarify the role of BMI in RRM outcomes.