Higher twin birth rates seen with increasing prepregnancy BMI, ART use

Key takeaways:

• More twin births occur among women with overweight, class I and II obesity vs. normal weight.
• Assisted reproductive technology mediated 25% of the links between women with obesity class I and II and twin births.

Twin birth rates rose with increasing prepregnancy BMI, and approximately 25% of the association can be explained by assisted reproductive technology use, according to study results published in JAMA Network Open.

“Previous studies from the United States and Denmark have shown an association between obesity and twinning rates, particularly with regard to dizygotic twins,” Jeffrey N. Bone, MSc, biostatistical lead in the department of biostatistics at the BC Children’s Hospital Research Institute and the department of obstetrics and gynecology at the BC Children’s and Women’s Hospital and Health Centre, Vancouver, British Columbia, Canada, and colleagues wrote. “Although the mechanism underlying this association is unclear, it is possible that the higher rates of twins among women with obesity are linked to higher rates of assisted reproductive technology, as women with overweight and obesity are more likely to have subfertility compared with those with normal BMI.”

Bone and colleagues conducted a retrospective cohort study using data from the British Columbia Perinatal Database Registry of 524,845 live birth and stillbirth deliveries at 20 weeks or longer gestation that occurred in British Columbia, Canada, from 2008 to 2020. Researchers evaluated prepregnancy BMI and assisted reproductive technology (ART) use in the cohort and examined their associations with twin birth rates.

In total, 392,046 women (median age, 31.4 years) had complete prepregnancy BMI data. Among these women, there were 15.8 twin deliveries per 1,000 deliveries. Twin rates per 1,000 deliveries were 11.9 among women with underweight, 15.1 among women with normal weight, 16 among women with overweight, 16 among women with obesity class I, 16.7 among women with obesity class II and 18.9 among women with obesity class III.

Women with underweight had 16% fewer twin births compared with women with normal weight (adjusted RR = 0.84; 95% CI, 0.74-0.95). In addition, twin deliveries were 14% higher among women with overweight (aRR = 1.14; 95% CI, 1.07-1.21), 16% higher among women with class I obesity (aRR = 1.16; 95% CI, 1.06-1.27), 17% higher among women with class II obesity (aRR = 1.17; 95% CI, 1.02-1.34) and 41% higher among women with class III obesity (aRR = 1.41; 95% CI, 1.19-1.66).

Compared with women with normal weight, women with overweight (aRR = 1.09; 95% CI, 1.05-1.14), obesity class I (aRR = 1.23; 95% CI, 1.16-1.31) and obesity class II (aRR = 1.26; 95% CI, 1.15-1.38) had higher rates of ART use. The proportion of conception by ART increased with increasing BMI, with ART associated with nearly a 12-fold higher rate of twin births (aRR = 11.8; 95% CI, 11.1-12.54). ART mediated about 25% of the associations between women with obesity class I and II and twin births, but this association was not mediated by ART among women with class III obesity, according to the researchers.

“Understanding the potential causes of twin pregnancy and delivery is essential from both clinical and public health perspectives, as women with twin pregnancy are more likely to experience complications, including severe maternal morbidity, preterm birth and stillbirth,” the researchers wrote.