Twin gestations have higher risks of fetal growth restriction and stillbirth than singleton gestations and are therefore monitored with serial ultrasounds for fetal growth because fetal growth restriction may affect antenatal testing and timing of delivery. Growth discordance between the fetuses, defined as a difference in birth weights of 10% to 20%, has been considered a marker of adverse outcomes. Growth discordance may suggest that the smaller twin is growth restricted (≤10th percentile for gestational age). The clinical significance of growth discordance in normally grown fetuses (>10th percentile for gestational age) is unclear. This retrospective, cohort study was undertaken to assess the association of growth discordance and adverse pregnancy outcomes in twins appropriately grown for gestational age.
The study included all patients with a twin gestation who underwent routine ultrasound at 15 to 22 weeks, with chorionicity determined at the earliest ultrasound. Discordance was defined as a 20% fetal weight difference or greater. Pregnancy outcomes compared between concordant and discordant twin pairs were stillbirth of 1 or both twins, preterm delivery less than 34 weeks or less than 28 weeks, and neonatal intensive care unit (NICU) admission. The analysis was stratified by chorionicity. Apgar scores, cord gases, and discharge summaries were obtained when available. Comparisons were made using the unpaired Student t test, Mann-Whitney U test, χ2 test, or Fisher exact test. Continuous variables were tested for normality visually and with the Kolmogorov-Smirnov test.
Of 2445 twin pairs, 1145 (46.8%) were included. Discordant twins were similar to concordant twins with regard to maternal age, race, history of stillbirth, hypertension, diabetes, and preeclampsia. No stillbirths occurred in the dichorionic twins discordant for birth weight. Discordant dichorionic twins were not at an increased risk of delivery at less than 34 weeks (34.9% vs 25.6%; RR = 1.4; 95% confidence interval [CI], 1.0–1.9) or less than 28 weeks (3.2% vs 2.8%; RR = 1.1; 95% CI, 0.3–4.8). The risk of either twin being admitted to the NICU was not increased in the discordant dichorionic twins. In monochorionic twins, discordant twins were at an increased risk of preterm delivery at less than 34 weeks (65.2% vs 26.4%; RR = 2.5; 95% CI, 1.7–3.6) and less than 28 weeks (34.8% vs 4.0%; RR = 8.8; 95% CI, 3.7–20.5). Monochorionic discordant twins were at greater risk of NICU admission (68.2% vs 23.3%; RR = 2.9; 95% CI, 2.0–4.3); this risk was not confined to the smaller twin. The risk of stillbirth was not increased among monochorionic discordant twins compared with concordant twins (4.6% vs 0.9%; RR = 5.2; 95% CI, 0.5Y54.7). Among both dichorionic and monochorionic twins, the most common indication for delivery at less than 34 weeks was spontaneous labor or spontaneous rupture of membranes. Discordant twins were more likely to be delivered because of nonreassuring fetal status or chorioamnionitis compared with concordant twins. Discordant dichorionic twin pairs were not at increased risk of Apgar scores of 3 or less at 5 minutes, umbilical cord pH of less than 7.10, umbilical cord base excess less than −12, requiring respiratory support, or intraventricular hemorrhage. Discordant monochorionic twin pairs were not at increased risk for the composite adverse neonatal outcome.
Discordant dichorionic twins were not at significantly increased risk of adverse perinatal outcomes, whereas discordant monochorionic twins were at increased risk of preterm birth, NICU admission, and intraventricular hemorrhage. In appropriately grown dichorionic twins, growth discordance may reflect a difference in the genetic potential for growth rather than a pathologic process. Because of genetic similarities in monochorionic twins, growth discordance is more likely a pathologic process. Appropriately grown monochorionic twins may benefit from increased antenatal surveillance in the presence of growth discordance.
Department of Obstetrics and Gynecology, University of Alabama at Birmingham, Birmingham, AL (L.M.H.); Department of Obstetrics and Gynecology, Washington University in St Louis, St Louis, MO (M.A.W., A.O.O., K.A.R., G.A.M., A.G.C.)