Background: Birthweight discordance is well studied, with less known about longitudinal inter-twin differences in foetal growth. Objective: To examine inter-twin per cent differences in EFW (EFW%), head (HC%) and abdominal circumference (AC%), and femur length (FL%) across gestation in dichorionic twin gestations and explore associated characteristics. Methods: Foetal biometrics were assessed by ultrasound and EFW calculated at ≤6 study visits among women with dichorionic twin pregnancies enrolled in the NICHD Fetal Growth Studies cohort (US, 2012-2013). Inter-twin per cent difference was defined: ([Sizelarger twin − Sizesmaller twin]/Sizelarger twin × 100). Linear mixed models evaluated per cent differences in foetal biometrics at 15 weeks and their change per week overall and by maternal/neonatal characteristics in unadjusted and adjusted models. Results: In 140 pregnancies, inter-twin per cent differences increased across gestation for EFW (0.18%/week, 95% confidence interval [CI] 0.10, 0.27), HC (0.03%/week, 95% CI 0.00, 0.06), and AC (0.03%/week, 95%CI −0.01, 0.08) but decreased for FL (−0.03%/week, 95% CI −0.09, 0.02). After adjustment, change in EFW% difference across gestation differed by pre-pregnancy body mass index (BMI [kg/m2]; underweight ['18.5]; normal weight [18.5-24.9]; overweight [25.0-29.9]; obese [≥30.0]; Pinteraction =.022); and conception method (in vitro fertilisation [IVF], intrauterine insemination, ovulation induction medication, donor egg/embryo, none; Pinteraction =.060). While EFW% difference increased with normal pre-pregnancy BMI (0.24%/week, 95% CI 0.12, 0.37), little change was noted with pre-pregnancy obesity (0.01%/week, 95% CI −0.15, 0.17). EFW% difference increased in conceptions without fertility treatments (0.23%/week, 95% CI 0.11, 0.34) but not IVF conceptions (−0.00%/week, 95% CI −0.16, 0.16). Similar patterns of differences across gestation were noted for HC% by conception method (Pinteraction =.026) and AC% by pre-pregnancy BMI (Pinteraction =.071); changes in HC% differed by parity (nulliparous, multiparous; Pinteraction =.004). Conclusions: EFW% difference increased across gestation in dichorionic twins, but remained stable with pre-pregnancy obesity or IVF conception, patterns mirrored for HC and AC. Research is needed to understand pathologic versus physiologic differential twin growth trajectories.
- estimated foetal weight
- foetal growth
- twin discordance
ASJC Scopus subject areas
- Pediatrics, Perinatology, and Child Health