Studies have reported controversial findings on the association between fetal growth restriction (FGR) or intertwin weight discordance and the risk of hypertensive disorders of pregnancy (HDP) in twin pregnancies. The aim of this study was to investigate the association between twin growth disorders and HDP. Twin pregnancies resulting in two live births at St George’s Hospital between 2000 and 2019 were included. FGR or small-for-gestational-age (SGA) at birth was assessed using singleton and twin reference charts. Intertwin discordance [(large birthweight − small birthweight)/(large birthweight) × 100%)] was calculated. Logistic regression models were performed. SGA (aOR 2.34, 95% CI 1.60–3.44, p < 0.001), intertwin discordance ≥25% (aOR 2.10, 95% CI 1.26–3.49, p = 0.004) and their co-existence (aOR 2.03, 95% CI 1.16–3.54, p = 0.013) were significantly associated with HDP. After adjusting for the known maternal risk factors of HDP and the intertwin discordance, SGA (using the twin charts) was the strongest independent risk factor associated with HDP (aOR 2.12, 95% CI 1.40–3.22, p < 0.001) and preeclampsia (aOR 2.34, 95% CI 1.45–3.76, p < 0.001). This study highlights that the presence of at least one SGA twin is significantly associated with HDP during pregnancy. Therefore, maternal blood pressure should be closely monitored in twin pregnancies complicated by SGA with or without intertwin discordance.

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Twin pregnancies are commonly assessed using singleton growth and birth weight reference charts. This practice has led to a significant number of twins labelled as small for gestational age (SGA), causing unnecessary interventions and increased risk of iatrogenic preterm birth. However, the use of twin-specific charts remains controversial. This study aims to assess whether twin-specific estimated fetal weight (EFW) and birth weight (BW) charts are more predictive of adverse outcomes compared to singleton charts. Centiles of EFW and BW were calculated using previously published singleton and twin charts. Categorical data were compared using Chi-square or McNemar tests. The study included 1740 twin pregnancies, with the following perinatal adverse outcomes recorded: perinatal death, preterm birth <34 weeks, hypertensive disorders of pregnancy (HDP) and admissions to the neonatal unit (NNU). Twin-specific charts identified prenatally and postnatally a smaller proportion of infants as SGA compared to singleton charts. However, twin charts showed a higher percentage of adverse neonatal outcomes in SGA infants than singleton charts. For example, perinatal death (SGA 7.2% vs. appropriate for gestational age (AGA) 2%, p < 0.0001), preterm birth <34 weeks (SGA 42.1% vs. AGA 16.4%, p < 0.0001), HDP (SGA 21.2% vs. AGA 13.5%, p = 0.015) and NNU admissions (SGA 69% vs. AGA 24%, p < 0.0001), when compared to singleton charts (perinatal death: SGA 2% vs. AGA 1%, p = 0.029), preterm birth <34 weeks: (SGA 20.6% vs. AGA 17.4%, p = 0.020), NNU admission: (SGA 34.5% vs. AGA 23.9%, p < 0.000). There was no significant association between HDP and SGA using the singleton charts (p = 0.696). In SGA infants, according to the twin charts, the incidence of abnormal umbilical artery Doppler was significantly more common than in SGA using the singleton chart (27.0% vs. 8.1%, p < 0.001). In conclusion, singleton charts misclassify a large number of twins as at risk of fetal growth restriction. The evidence suggests that the following twin-specific charts could reduce unnecessary medical interventions prenatally and postnatally.

What are the novel findings of this work?We present a comparison of the ability of twin-specific vs singleton estimated-fetal-weight and birth-weight reference charts to predict adverse neonatal outcome in small-for-gestational-age (SGA) twins. Infants classified as SGA according to the twin-specific charts, but not those classified as SGA according to the singleton charts, had a significantly increased risk of adverse neonatal outcome compared with appropriate-for-gestational-age infants. What are the clinical implications of this work?This study presents further evidence that twin-specific charts perform better than singleton charts in the prediction of adverse neonatal outcome in twin pregnancies. The use of twin-specific charts may reduce misclassification and improve identification of infants who may not be at increased risk of adverse outcome despite being labeled as SGA using singleton standards.