A new study revealed that dichorionic twins may accumulate less fat tissue compared with singletons much earlier in pregnancy than previously thought—as early as 15 weeks' gestation. The findings could provide new insights into twin fetal development and may support the concept that twin growth patterns represent an evolutionary adaptation rather than pathologic growth restriction.
In the study, in JAMA Pediatrics, researchers from the National Institute of Child Health and Human Development and collaborating institutions found that twin fetuses had 2.7% to 4.2% less thigh fat tissue throughout pregnancy compared with singletons, with statistically significant differences detectable from the beginning of the study's observation period.
"Twins had proportionally less fat tissue accumulation in utero compared with singletons as early as 15 weeks' gestation, when competition for nutritional resources was low," reported lead study author Jessica L. Gleason, PhD, and her colleagues. "Persistent findings of smaller twin sizes and less fat accumulation across pregnancy support the concept of an early evolutionary adaptive process in otherwise uncomplicated dichorionic twin growth," they added.
The prospective cohort study analyzed three-dimensional ultrasound data collected between 2009 and 2013 from 315 twin and 2,604 singleton fetuses across 12 U.S. clinical centers. The researchers measured multiple parameters of fetal body composition, including abdominal area, maximum abdominal subcutaneous tissue thickness, fractional thigh volume, and separate measurements of lean and fat thigh components.
The findings revealed statistically significant differences between twins and singletons across all measurement parameters. The abdominal area was notably smaller in twins compared with singletons from 25 to 37 weeks' gestation, with the difference increasing from –48.6 mm² (95% confidence interval [CI] = –102.2 to –5.1) at 25 weeks to –480.5 mm² (95% CI = –677.2 to –283.5) at 37 weeks.
Differences in thigh measurements were detectable from the earliest observation point at 15 weeks. The mean fractional thigh volume difference was –0.11 cm³ (95% CI = –0.16 to –0.07) at 15 weeks, expanding to –7.55 cm³ (95% CI = –11.76 to –3.34) by 37 weeks. Fractional fat thigh volume showed similar patterns, with twins having –0.08 cm³ (95% CI = –0.12 to –0.05) less fat tissue at 15 weeks, increasing to –5.60 cm³ (95% CI = –8.37 to –2.82) by 37 weeks.
These findings provided new perspectives on the understanding that twin growth restriction primarily occurs during the third trimester as a result of placental insufficiency. Previous studies using two-dimensional ultrasound measurements identified growth deceleration in twins beginning around 26 to 32 weeks.
"Our results support these calls [for twin-specific fetal growth charts]; however, more research is needed to evaluate both longitudinal growth patterns and health outcomes in twins to determine if their lower fat accumulation and smaller size throughout gestation translate to any long-term difference in size or increased risk for short- and long-term morbidity," the study authors noted.
The early detection of differences at 15 weeks, when uterine compliance is high and competition for nutritional resources is minimal, suggested that growth-regulatory processes may be programmed early in twin pregnancy development.
The study also indicated that twin growth restriction may differ fundamentally from fetal growth restriction observed in singletons. While singleton growth restriction is typically diagnosed in the third trimester and associated with placental insufficiency, the twin pattern appeared to represent a different physiologic adaptation beginning much earlier in gestation.
The researchers suggested that "twin smallness" may be a normal adaptive mechanism rather than a pathologic process—a distinction with potential implications for clinical assessment of twin pregnancies and the potential development of twin-specific fetal growth references.
Disclosures can be found in the study.