High BMI in both men and women associated with Fertility and Miscarriage Risk: JAMA

A new study conducted in Rotterdam, the Netherlands, has found a link between the body mass index (BMI) in both men and women and its significant effect on fertility outcomes like conception and the risk of miscarriage. The research highlights that BMI beyond the normal range in men and women can reduce the chances of conception and increase the risk of pregnancy loss, suggesting that managing weight before planning pregnancy is important for improving reproductive health. The study results were published in the journal JAMA Network Open. 

Obesity is a global pandemic. It has posed major challenges in both men and women. In women obesity is associated with multiple adverse health outcomes like hypertensive disorders, hyperglycemia, adverse birth outcomes, lower fertility, increased assisted pregnancy, and cardiovascular outcomes. Apart from women, BMI in men is also associated with adverse fertility outcomes. Hence, researchers conducted a study on a hypothesis that preconception BMI and early-pregnancy BMI outside of the normal category, not only in women but also in men, are associated with time to pregnancy and miscarriage.

A population-based cohort study was carried out between August 2017 and July 2021, involving 3,604 women and their partners from the preconception period through birth. Researchers analyzed BMI data measured in early pregnancy or preconception and examined its relationship to key reproductive outcomes. The main outcomes of measurement include fecundability (the likelihood of conceiving within a month), subfertility (taking over 12 months to conceive or needing assisted reproductive technology), and miscarriage (defined as pregnancy loss before 22 weeks of gestation) which were assessed by questionnaires by an obstetric caregiver. 

Findings:

• The study found that higher BMI in both women and men was negatively associated with fecundability.
• For every unit increase in BMI, the probability of conceiving decreased slightly, with a fecundability ratio (FR) of 0.98 in women and 0.99 in men.
• Overweight (BMI 25-29.9) and obese (BMI 30 and above) women had significantly lower chances of conception compared to those with normal weight, with FRs of 0.88 and 0.72, respectively.
• Abnormal BMI levels are linked to subfertility.
• Underweight women (BMI below 18.5) had nearly twice the odds of experiencing subfertility compared to those with normal weight (odds ratio [OR] 1.88).
• Overweight (OR 1.35) and obesity (OR 1.67) were also associated with higher risks of subfertility in women.
• In men, obesity was similarly linked to subfertility, with an OR of 1.69, highlighting that excess weight in both partners can hinder the chances of natural conception.
• Miscarriage risks were another key area of concern with overweight (OR 1.49) and obese women (OR 1.44) having an increased likelihood of experiencing a miscarriage before 22 weeks of gestation compared to those with a normal BMI. 

While obesity has long been known to affect women’s fertility, this study confirms that BMI affects fertility in men as well suggesting that optimizing BMI before conception is important in improving reproductive outcomes for couples. The study results highlight the importance of a healthy weight for both men and women during the preconception period. As more couples' face fertility challenges, the study points to weight management as a practical strategy for boosting fertility and minimizing miscarriage risk.

Further reading: 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