Preconception BMI drives fertility struggles and miscarriage risk

Preconception BMI drives fertility struggles and miscarriage risk | Image Credit: © Prostock-studio - © JProstock-studio - stock.adobe.com.

Preconception body mass index (BMI) outside of the normal category in men and women is associated with time to pregnancy and miscarriage, according to a recent study published in JAMA Network Open.¹

Obesity during pregnancy has been associated with multiple adverse birth outcomes, including increased risks of miscarriage and pregnancy loss.² However, investigators have hypothesized BMI beyond the extremes of obesity, alongside BMI in both women and men, may impact fertility outcomes and miscarriage risk.¹

“A better understanding of the separate and combined associations of BMI in women and men with fertility and miscarriage outcomes is needed to develop novel targeted population strategies to optimize BMI from the preconception period onward,” wrote investigators.

BMI Measurement and definitions

The study was conducted to evaluate the impact of BMI in women and men with fertility and birth outcomes. Participants included women and their partners aged at least 18 years, living in Rotterdam, and pregnant or trying to conceive between August 9, 2017, and July 1, 2021.

Patients underwent height and weight measurements without wearing shoes or heavy clothing. In men, these assessments were performed during a first-trimester visit. In women who were pregnant, a questionnaire was used to obtain prepregnancy weight, with prepregnancy BMI measured by substituting prepregnancy weight with measurements at first-trimester visits.

Underweight BMI was defined as under 18.5, normal weight as 18.5 to 24.9, overweight as 25 to 29.9, and obesity as 30 or greater. Questionnaires in preconception and early pregnancy were administered to evaluate time to pregnancy and mode of conception. Assisted reproductive technology use, including in vitro fertilization, was also reported.

Assessing fertility and pregnancy outcomes

The start date of seeking conception to the last date of study participation was used to determine the duration of actively pursuing pregnancy in patients unable to conceive. Fecundability, defined as the odds of conception within 28 days, was also reported.

Categories of time to pregnancy included 12 months or less and over 12 months, defined as fertile and subfertile, respectively. Pregnancy loss before 22-weeks’ gestation was defined as miscarriage. Covariates included age, ethnicity, and greatest education level.

There were 3033 pregnancy or preconception episodes across women aged a median 31.6 years and with a median BMI of 23.5 included in the analysis. In men, the median age and BMI were 33.4 years and 24.9, respectively. Of episodes among women, 17.8% were subfertile and 11.3% led to miscarriage. Overall, the median time to pregnancy was 3.7 months.

Negative association for BMI and fecundability

A decrease in fecundability was identified for every unit increase in BMI among both women and men, with a fecundability ratio (FR) of 0.98 for the former and 0.99 for the latter. Additionally, an FR of 0.88 was reported for overweight vs normal weight status, alongside 0.72 for obesity vs normal weight.

In men, overweight and obesity also decreased fecundability, with an FR of 0.89. Women and men also reported FRs of 1.04 and 1.03, respectively, for subfertility with increased BMI. The FRS for subfertility among women with underweight, overweight, and obesity vs normal weight were 1.88, 1.35, and 1.67, respectively.

An increased probability of miscarriage per week was also reported in normal weight vs overweight women, with a hazard ratio of 1.43. However, no other associations between BMI in women and men and miscarriage odds were reported.

Implications

Odds ratios for miscarriage were 1.49 in overweight women and 1.44 in women with obesity. Overall, the results highlighted associations of BMI outside the normal range in women and men with reduced fecundability, subfertility, and increased miscarriage odds.

“Optimizing BMI from the preconception period onward in women and men might be an important strategy to improve fertility and pregnancy outcomes,” wrote investigators.

References

1. Boxem AJ, Blaauwendraad SM, Mulders AGMGJ, et al. Preconception and early-pregnancy body mass index in women and men, time to pregnancy, and risk of miscarriage. JAMA Netw Open. 2024;7(9):e2436157. doi:10.1001/jamanetworkopen.2024.36157
2. Magnus MC, Hockey RL, Håberg SE, Mishra GD. Pre-pregnancy lifestyle characteristics and risk of miscarriage: the Australian Longitudinal Study on Women's Health. BMC Pregnancy Childbirth. 2022;22(1):169. doi:10.1186/s12884-022-04482-9