Twins show slower growth than singletons starting at 15 weeks
New research reveals that twin fetuses have reduced fat and muscle mass compared to singletons as early as 15 weeks of gestation, offering fresh insight for prenatal care.
Twins show slower growth than singletons starting at 15 weeks | Image Credit: © Martin Valigursky - © Martin Valigursky - stock.adobe.com.
Introduction
Twins start out smaller in pregnancy than what was previous known, according to a recent study published by the National Institutes of Health (NIH).1
The data was obtained from prior ultrasound studies evaluating fetal development, including 3D ultrasound scans of 2,604 singleton pregnancies and 315 twins. Twins were found to have reduced fat tissue and muscle mass compared with singletons starting at 15-weeks’ gestation.
“If results are confirmed by additional research, the findings could provide information to guide physicians in monitoring and managing twin pregnancies,” wrote investigators.
Study purpose and background
The study was conducted to expand on work using 2D ultrasound parameters.2 Participants with singleton pregnancies received up to 5 ultrasound scans and twin pregnancies up to 6. Higher odds of nonsmoking, having a body mass index (BMI) under 30, and not presenting with major chronic conditions were reported in the singleton cohort.
Only dichorionic twin pregnancies were included in the twin pregnancy cohort, determined by the presence of 2 gestational sacs with a thick intervening membrane. First-trimester ultrasonography was used to determine participants’ menstrual age. An interview was performed at enrollment to obtain maternal demographic data.
Measurement
Parameters compared between singleton and twin pregnancies included abdominal area, maximum abdominal subcutaneous tissue thickness (MASCTT), fractural thigh volume (TVol), fractural lean thigh volume (FLTVol), fractional fat thigh volume (FFTVol), midtigh area (MTA), midthigh lean area (MTLA), and midthigh fat area (MTFS).
An FFTVol to TVol ratio was also compared. Measurements were obtained from trained sonographers.
Demographic and growthdifferences
A slightly older age was reported among women with twin deliveries vs singletons, alongside a higher prepregnancy BMI. This population also more often identified as non-Hispanic White.
Mean gestational ages at delivery were 35.2 weeks for twin pregnancies and 39.2 weeks for singleton pregnancies. Having at least 3 ultrasound visits across pregnancy was reported in 88.3% and 90.6%, respectively.
Starting at 15 weeks, significantly reduced mean abdominal measurements were reported in twins vs singletons. These differences underwent nonsignificant narrowing at between 19- and 22-weeks’ gestation, then were consistently larger from 25- to 37-weeks’.
A slight reduction in the abdominal area was observed at 15 weeks, with a mean difference of 37.6 mm2. At 25 weeks, this difference rose to 48.6 mm2, then to 480.5 mm2 at 37 weeks. A similar trend was observed for MASCTT, with a difference of 0.13 at 27 weeks vs 0.40 at 37 weeks.
Reduced thigh differences
Twins also had smaller mean thigh measurements compared to singletons across all weeks of gestation from 15 to 37. For TVol, this reduction was 0.11 cm3 at week 15 and 7.55 cm3 at week 37. FLTVol also showed a difference at 15 weeks, but this difference was not significant from week 19 to week 22. At 23 weeks, twins had a significant reduction of 0.17 cm3.
Similar patterns were also reported for MTA, MTLA, and MTFA. Twins also had reduced thigh fat compared to singletons, with a reduction of approximately 2% observed in fractional fat volume throughout gestation.
Conclusion and implications
Additionally, twins had consistently reduced thigh fat area, with the greatest difference of 4.6% observed at week 15. Overall, the data highlighted identifiable reductions in sizes among twins vs singletons by 15-weeks’ gestation.
“Our findings also indicate that the growth of dichorionic twins may start to differ from that of singletons earlier than the third trimester, as repeatedly observed in previous studies using traditional 2-D ultrasound measurements,” concluded investigators.
References
- Twins grow more slowly in early pregnancy than previously thought. National Institutes of Health. April 7, 2025. Accessed April 8, 2025. https://www.nih.gov/news-events/news-releases/twins-grow-more-slowly-early-pregnancy-previously-thought.
- Gleason JL, Lee W, Chen Z, et al. Fetal body composition in twins and singletons. JAMA Pediatr. 2025. doi:10.1001/jamapediatrics.2025.0116
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