A recent study published in JAMA Network Open has identified a significant connection between body mass index (BMI) outside of the normal range and both time to pregnancy and the risk of miscarriage in both women and men during the preconception and early-pregnancy stages. This research is particularly relevant given the growing public health concern regarding obesity among reproductive-aged women.
Obesity is associated with several risks during pregnancy, including gestational hypertension, adverse birth outcomes, and cardiovascular issues. While there is increasing evidence linking BMI to early-pregnancy outcomes, understanding the distinct and combined effects of BMI in both genders on pregnancy outcomes remains limited.
To address this gap, researchers analyzed data from the Generation R Next Study, a population-based prospective cohort study conducted in Rotterdam. The study aimed to identify preconception and early-pregnancy determinants of various pregnancy outcomes. Participants included both women and their partners, all aged 18 and older, who were either trying to conceive or currently pregnant.
Of the participants, 33.2% were in the preconception phase, while 52.8% were in the first trimester of pregnancy. Individuals were excluded from the study if they lacked BMI data, time to pregnancy data, or if they had not achieved pregnancy by the end of the study period.
BMI was assessed through direct measurements of height and weight, taken without shoes or heavy clothing. For women, measurements were taken at enrollment and during follow-up visits, while men were assessed during the first trimester. BMI categories were defined as follows: underweight (BMI < 18.5), normal weight (BMI 18.5-24.9), overweight (BMI 25-29.9), and obese (BMI ≥ 30).
Time to pregnancy was evaluated using questionnaires administered during the preconception and early-pregnancy phases. The start date for actively pursuing pregnancy and the first day of the last menstrual period were used to calculate this time. A time to pregnancy of 12 months or less was classified as fertile; those who did not conceive within that period or who required assisted reproductive technology were classified as subfertile.
Miscarriage was defined as a pregnancy loss occurring before 22 weeks of gestation, with data gathered from obstetrical caregivers. Additional covariates included the participants’ age, ethnicity, and highest level of education.
The study included 3,033 episodes for time-to-pregnancy analysis among women, who had a median age of 36.1 years and a median BMI of 23.5. For men, 2,288 episodes were included, with a median age of 33.4 years and a median BMI of 24.9. For miscarriage analysis, the study comprised 2,770 episodes among women (median age 31.5 years, median BMI 23.5) and 2,189 episodes among men (median age 33.5 years, median BMI 25).
The median time to pregnancy reported was 3.7 months, with 17.8% of women classified as subfertile and 11.3% experiencing a miscarriage. The fecundability ratio (FR) was 0.98 for women and 0.99 for men.
The findings indicated that women with overweight and obesity experienced a reduction in fecundability compared to those with normal weight. Higher BMI was associated with increased odds of subfertility, with odds ratios (OR) of 1.04 for women and 1.03 for men. Specifically, the ORs for subfertility among women were 1.88 for underweight, 1.35 for overweight, and 1.67 for obesity compared to normal weight.
Additionally, being overweight was linked to a higher risk of miscarriage, with a hazard ratio of 1.43. The authors noted that the small number of underweight and obese women in the study may limit the generalizability of these findings.
In conclusion, the study indicates that BMI outside of the normal range is associated with reduced fecundability, increased subfertility, and heightened miscarriage risk in both women and men. The researchers advocate for efforts to optimize BMI from preconception onward as a potential strategy to improve pregnancy outcomes.
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