Associations between estimated foetal weight discordance and clinical characteristics within dichorionic twins: The NICHD Fetal Growth Studies

Abstract

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.

Making sense of foetal growth: What is normal in twins?

“…6 This study by Amyx et al found that maternal underweight and IVF were both associated with small for gestational age in twins. 4 Despite the evidence, there is much disagreement and controversy regarding the definition of what is reduced foetal growth among twins and whether twin-specific or singleton growth charts should be utilised to determine twin growth trajectory or define pathologic small size. 1 Twin charts are recommended by some, while others use singleton charts.…”

“…In this issue of Paediatric and Perinatal Epidemiology, Amyx and colleagues used data from the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) Fetal Growth Study‐Twins to investigate inter‐twin differences in foetal growth by considering pre‐existing maternal factors that may alter growth trajectory such as maternal age, race, pre‐pregnancy BMI, parity, fertility treatment/method of conception, infant sex, and zygosity. The NICHD Fetal Growth Study was designed to assess foetal growth trajectories, foetal growth restriction, and maternal biomarkers of foetal growth in DCDA twins and compare these with singletons (2).…”

“…Poor placental function resulting in poor intrauterine foetal growth be attributable to maternal smoking, hypertensive diseases, maternal weight (either low or overweight or obesity) and nutritional disorders . This study by Amyx et al found that maternal underweight and IVF were both associated with small for gestational age in twins . Despite the evidence, there is much disagreement and controversy regarding the definition of what is reduced foetal growth among twins and whether twin‐specific or singleton growth charts should be utilised to determine twin growth trajectory or define pathologic small size .…”

“…However, uncertainties remain as to optimal surveillance, what is considered pathological and when to time delivery. The study by Amyx and colleagues helps identify maternal characteristics that may warrant increased surveillance to ensure optimal pregnancy outcomes …”

“…Overall, 90% of twins were dizygotic, about half (46.4%) of women had fertility treatment and two‐thirds were born at 35 weeks or later in gestation. The study found that estimated foetal weight percentage (EFW % ) discordance in DCDA twins increased over gestation by 0.18% (95% confidence interval [CI] 0.10, 0.27) from 6% at 15 weeks to 9.6% at 35 weeks, with minimal difference in EFW % at 15 weeks or change in EFW % across gestation by maternal groups. Maternal factors that influenced EFW discordance were pre‐pregnancy BMI and method of conception.…”