ART's mediating role in BMI and twin births


A recent study unveiled how prepregnancy body mass index influences twin births, with assisted reproductive technology use as a noteworthy mediator.

ART's mediating role in BMI and twin births | Image Credit: © New Africa - © New Africa -

ART's mediating role in BMI and twin births | Image Credit: © New Africa - © New Africa -

Prepregnancy body mass index (BMI) is associated with the odds of twin births, but is mediated by assisted reproductive technology (ART) use, according to a recent study published in JAMA Network Open.


  1. There has been a global increase in the rate of twin births over the years, rising from 9.1 per 1000 deliveries between 1980-1985 to 12 per 1000 between 2010-2015.
  2. The study was a retrospective cohort study conducted in British Columbia, Canada, encompassing women who delivered at 20 weeks' or longer gestation between April 2008 and March 2020.
  3. Women with higher prepregnancy BMIs, particularly those classified as overweight or obese, demonstrated increased rates of twin deliveries. However, this association was mediated by the use of assisted reproductive technology (ART).
  4. ART, particularly in vitro fertilization, has a significant impact on the likelihood of twin births. Its use was linked to a 12-fold increase in twin deliveries. Furthermore, ART accounted for 4% of twin births attributed to overweight or obese prepregnancy BMI.
  5. There's a correlation between increasing BMI categories and increased rates of ART use. For instance, while ART rates were lowest among underweight women and those with class 3 obesity, they increased substantially in overweight, obesity class 1, and obesity class 2 categories.

An increase in the rate of twin births has been observed worldwide, from 9.1 per 1000 deliveries between 1980 and 1985 to 12 per 1000 between 2010 and 2015. A rise in dizygotic twinning has been the primary source of this increase, leading to health concerns because of associated risks of maternal, fetal, and infant complications.

Increased use of ART has been linked to the rise in twinning rates, contributing to 10.6% of multiples. Overweight and obesity, impacting 30% to 50% of reproductive-aged women, is associated with higher odds of using ART.

To determine the association between prepregnancy BMI and twin delivery and how ART use impacts this association, investigators conducted a retrospective cohort study. Participants included women who delivered at 20 weeks’ or longer gestation from April 2008 to March 2020 in British Columbia, Canada.

Exclusion criteria included delivery under 20 weeks’ gestation, missing gestational age and plurality information, and pregnancy termination. The British Columbia Perinatal Database Registry was evaluated for individual level data including maternal demographic characteristics, prepregnancy BMI, pregnancy morbidity, chronic medical conditions, delivery, and newborn hospitalizations.

Prepregnancy BMI, determined using self-reported height and prepregnancy weight, was the primary exposure of the analysis. Underweight was defined as BMI under 18.5, normal BMI as 18.5 to 24.9, overweight as 25 to 29.9, obesity class 1 as 30 to 34.9, obesity class 2 as 35 to 39.9, and obesity class 3 as 40 or greater.

Twin delivery was the primary outcome of the analysis, mediated by ART. In vitro fertilization use during conception was used to determine ART.

There were 392,046 women with complete prepregnancy BMI data available, 5.7% of whom were underweight, 59.1% had normal BMI, 21.4% overweight, 8.5%obesity class 1, 3.4% class 2, and 1.9% class 3. 

Similar maternal age and height were observed across BMI groups except for a slightly younger age among underweight individuals.Increased parity was observed among patients with obesity, and smoking during pregnancy was more common among underweight and obese patients. 

ART rates were 22.1 per 1000 deliveries among underweight women and 20.4 per 1000 deliveries among women with class 3 obesity, which were the lowest reported rates. Rates of ART use increased by 9%, 23%, and 26% among patients with overweight, obesity class 1, and obesity class 2, respectively.

Twin deliveries were reported in 15.8 per 1000 deliveries overall. A rate of 14.4 per 1000 deliveries was seen in women with a normal BMI. In women with overweight, obesity class 1, and class 2, this rate significantly increased to 16, 16, and 16.7 per 1000 deliveries, respectively.

A reduction in twin deliveries was reported in underweight women, at 11.3 per 1000 deliveries. The greatest rate was observed in women with class 3 obesity, at 18.9 per 1000 deliveries.

Of twin births, 4% were attributed to overweight or obese prepregnancy BMI. ART use was associated with a 12-fold increase in twin delivery.

These results indicated an association between overweight and obesity with increased twin delivery rates. This association was mediated by increased ART use among these individuals, except for those with a BMI of 40 or higher.


Bone JN, Joseph KS, Magee LA, et al. Obesity, twin pregnancy, and the role of assisted reproductive technology. JAMA Netw Open. 2024;7(1):e2350934. doi:10.1001/jamanetworkopen.2023.50934

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