Swati Mishra
April 7, 2025: Multiple pregnancy, where a mother carries more than one baby in the womb, comes with its own set of complications, such as premature birth, preeclampsia, and fetal growth restriction. The latest study by researchers at the National Institutes of Health (NIH) revealed that twins start out much smaller than singletons in early pregnancy than previously known.
The ultrasound study found that twins have less fat tissue and less muscle mass than singletons beginning at 15 weeks of pregnancy. The researchers did a meta-analysis of previous 3D ultrasound studies of 2,604 singleton pregnancies and 315 twins (with individual placentas). The study did not include monochorionic twin pregnancies where the babies share one placenta.
The researchers found that at the end of 14 weeks, the twins had leaner thighs with less fat volume. They had a lower thigh fat-to-lean ratio than singletons, having proportionately 2.7 to 4.2% less fat from 15 to 37 weeks. Previous studies dependent on 2D ultrasounds found the differences in growth only at 28 to 40 weeks, that is, in the third trimester.
This discovery of early growth differences in twin vs singleton pregnancies will allow doctors to better monitor multiples’ growth patterns.
Increase in the number of twin pregnancies
Data from developed countries demonstrates an increase in the incidence of twinning of between 28 and 59% over the past two decades. This increase has been linked largely to the increased utilization of artificial reproductive technologies (ART) and, to a lesser extent, the increase in mean maternal age over this period of time
Also, the increased incidence of twinning has neonatal care implications, as it is well established that twins have an increased incidence of common neonatal problems and this study is bound to aid in finding better outcomes for such pregnancies.
A large cohort study done in Belgium with 12,021 singleton and 3,108 twin births reported that, in comparison to singleton births, twin birth had a significantly increased risk of preterm delivery [relative risk (RR), 6.88], LBW (RR, 8.58), and very LBW (RR, 7.29) when infants were matched for maternal age, parity, fetal sex, and year of birth.
Implications of the study
Generally, the growth complications occur due to the sharing of nutrients and organic resources amongst multiples. But scientists do not think the twins’ early small size resulted from increased competition for nutrients as their placentas should have been developed enough to supply needed resources until the third trimester. Rather, they believe that the twins’ small size early in pregnancy could be a mechanism to compensate for the increased resources of two fetuses later in pregnancy.
While twins are often born smaller and may experience a higher risk of preterm delivery and related complications, some studies suggest that once they reach a certain age, they may have similar or even better health outcomes compared to singletons.
For example, some studies have found that twins have a lower risk of depression and suicide than singletons and may even have a higher life expectancy.
Research gaps
The findings suggest that current ultrasound methods used to monitor fetal growth in singletons may not be sufficient for twins. The American Journal of Obstetrics & Gynecology suggests that using twin-specific growth charts, rather than singleton charts, may be more appropriate for monitoring twin pregnancies.
More research is needed to understand the long-term developmental outcomes of twins, especially those classified as small for gestational age (SGA), using singleton references. There is a need for studies that compare neurodevelopmental outcomes in SGA twins using both singleton and twin-specific references.