Preconception BMI linked to fertility and miscarriage risks

Preconception BMI outside the healthy range is linked to longer time to pregnancy and higher miscarriage risk, new research has revealed.

The study followed 3,033 pregnancy or preconception episodes among couples in Rotterdam between August 2017 and July 2021, examining how body mass index (BMI) – weight relative to height – affects fertility.

Women had a median age of 31.6 years and men 33.4. BMI categories were underweight (under 18.5), normal (18.5–24.9), overweight (25–29.9) and obese (30 or greater).

Fecundability – the chance of conceiving in one menstrual cycle – declined with each BMI unit for both sexes.

Women with obesity had a 28 per cent lower chance of conceiving than women of normal weight, while overweight women had a 12 per cent reduction.

Overweight and obese men had an 11 per cent reduction compared with normal-weight men.

The researchers wrote: “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 optimise BMI from the preconception period onward.”

Of the cases studied, 17.8 per cent of women were classed as subfertile, meaning conception took more than 12 months. In addition, 11.3 per cent of pregnancies ended in miscarriage, defined as loss before 22 weeks’ gestation.

Underweight women had nearly twice the risk of subfertility compared with normal-weight women.

Overweight women faced a 35 per cent higher risk, while women with obesity had a 67 per cent higher risk.

Miscarriage risks were also higher outside the healthy BMI range: overweight women had a 49 per cent higher risk and women with obesity a 44 per cent higher risk than normal-weight women.

The median time to pregnancy across all participants was 3.7 months. The study also recorded use of assisted reproductive technology, including in vitro fertilisation, among couples struggling to conceive.

The investigators concluded: “Optimising BMI from the preconception period onward in women and men might be an important strategy to improve fertility and pregnancy outcomes.”