Perinatal & Pediatric
Fetal body composition in twins and singletons – are twins programmed to be smaller? Jessica L. Gleason* Jessica Gleason Wesley Lee Zhen Chen Kathryn A. Wagner Daniel He William A. Grobman Roger Newman Seth Sherman Edward Chien Robert Gore-Langton Luis Goncalves Katherine L. Grantz
Background: Twins and singletons have different fetal growth trajectories, with twin growth significantly slowing beginning in the third trimester of gestation relative to singletons. These differences may represent normal physiological adaptation in otherwise uncomplicated twin pregnancies. However, there is a lack of information about the sequential progression of fetal soft tissue development in both singleton and twin pregnancies.
Methods: In the NICHD Fetal 3D Study, serial abdominal and thigh measurements were obtained using three-dimensional ultrasonography in dichorionic twins (n=306) and singletons (n=2,525). Using linear mixed effects models, we compared marginal means at each week of gestation (15-36) for the following fetal soft tissue parameters: abdominal area, maximum subcutaneous tissue thickness (MSCTT), fractional thigh volume (TVol), fractional fat thigh volume (FFTVol), fractional lean thigh volume (FLTVol) and a FFTVol/TVol ratio, adjusting for relevant covariates.
Results: Twin abdominal measures were slightly larger than singletons from 17-26 weeks for area (2.1-41.3 mm2) and 18-24 weeks for MSCTT (0.04-0.07 mm) and becoming smaller thereafter (area=24.1-303.1 mm2; MSCTT=0.04-0.35 mm). Beginning at 15 weeks, thigh volumes were smaller for twins (diffTVol= -0.09 cm3, 95% CI: -0.13, -0.05; and diffFFTVol= -0.07 cm3, CI: -0.12, -0.02) relative to singletons, persisting through 36 weeks (diffTVol= -6.77 cm3, CI: -10.0, -3.52 and diffFFTVol= -4.98 cm3, CI: -7.93, -1.71). FLTVol was also smaller for twins in all weeks of gestation. For the FFTVol to TVol ratio, twins still had a 1.3-3.2% smaller fat percentage for all weeks of gestation compared to singletons.
Conclusions: Despite third trimester twin growth deceleration, our discovery that twin thigh volumes consistently remain smaller from 15 weeks of gestation implies that twin growth adaptation may be predetermined in early pregnancy at a time when there is less competition for in-utero resources.