Media Advisory

Monday, April 7, 2025

Twins grow more slowly in early pregnancy than previously thought

NIH findings could lead to more efficient monitoring of twin pregnancies.

What

Twins—smaller at birth, on average, than singletons—start out smaller in pregnancy than was previously known, according to a study by researchers at the National Institutes of Health (NIH). The ultrasound study revealed that twins have less fat tissue and less muscle mass than singletons beginning at 15 weeks of pregnancy. If results are confirmed by additional research, the findings could provide information to guide physicians in monitoring and managing twin pregnancies.

Researchers analyzed data from previous ultrasound studies of fetal development in the uterus.  Included in the analysis were 3D ultrasound scans of 2,604 singleton pregnancies and 315 twins. All twin pairs had individual placentas. Twin pairs who shared a placenta were not included in the analysis.

Beginning at 15 weeks, thigh volumes were significantly smaller for twins than singletons. Twins had smaller fat and lean thigh volumes than singletons. They also 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 of twin growth in the womb involved 2D ultrasound scans and had documented decreased twin growth relative to singletons only in the early 3rd trimester (28 to 40 weeks).

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 hypothesize that twins’ small size early in pregnancy could be an adaptation to accommodate the need for increased resources of two fetuses later in pregnancy.

The study was conducted by Jessica Gleason, Ph.D. and Katherine Grantz, M.D., of NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development, and their colleagues. It appears in JAMA Pediatrics.

Who

Jessica Gleason, Ph.D., a perinatal epidemiologist, is available for comment.

Article

Gleason JL, et al. Fetal body composition in twins and singletons. JAMA Pediatrics. DOI: https://doi.org/10.1001/jamapediatrics.2025.0116 (2025)

About the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD): NICHD leads research and training to understand human development, improve reproductive health, enhance the lives of children and adolescents, and optimize abilities for all. For more information, visit https://www.nichd.nih.gov.

 

About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit www.nih.gov.

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