Twin fetuses are born earlier and have slower growth than singletons. It is uncertain as to what degree this is pathological or a physiological adaptation.

We set out to develop a customized chart for twin pregnancy and compare it with the corresponding singleton chart in its ability to assess risk of adverse perinatal outcomes.

The cohort consisted of 8457 twin pregnancies (16,914 fetuses) without information on chorionicity, recorded during routine care in 127 UK hospitals. We performed a mixed-effects linear regression analysis to calculate customized coefficients for maternal height, weight, parity, and ethnic origin and to determine pregnancy-specific optimal weights at 37 weeks. This weight was linked to a proportionality curve derived from serial scans of twin pregnancies with normal outcomes. We compared the new customized standard for twins with that for singletons by calculating rate of small for gestational age (<10th centile)–associated risk of stillbirth and a set of adverse neonatal outcomes (need for resuscitation, Apgar score <7 at 5 minutes, admission to neonatal intensive care unit, or neonatal death), using generalized estimating equations and odds ratios and 95% confidence intervals. The effect of customization on small for gestational age and large for gestational age rates in different ethnic groups was compared with 3 uncustomized twin-specific charts.

The same maternal physiological characteristics (maternal height, early pregnancy weight, parity, and ethnic origin) were found to affect the twin weight standard as singletons, and high body mass index had similarly a significantly negative effect on weight at birth. The average optimal weight at 37+0 weeks for the same maternal characteristics was 389 g less for a twin compared to a singleton fetus. Customized twin and singleton standards designated as small for gestational age 13.4% and 44.2% of twins, respectively. Small for gestational age by customized twin standard had a higher risk of stillbirth (odds ratio, 7.2; confidence interval, 4.8–10.9) than small for gestational age by singleton standard (2.8; 1.9–4.1), and small for gestational age by singleton but not by twin standard (68.9% of all singleton standard small for gestational age cases) had no increase in stillbirth risk. Neonates small for gestational age by customized twin standard had an increased need for resuscitation (odds ratio, 1.3; confidence interval, 1.1–1.7), lower Apgar score (<7) at 5 minutes (odds ratio, 1.8; confidence interval, 1.2–2.6), higher admission rate to neonatal intensive care unit (odds ratio, 1.3; confidence interval, 1.0–1.6), and increased risk of neonatal death (odds ratio, 5.4; confidence interval, 1.3–23.5), while neither of these risks were increased with singleton standard. Small for gestational age rates by population-based twin standards were higher than by the customized twin standard, but the additional cases were not or only weakly associated with stillbirth risk.

Use of a singleton standard for twins results in a 3 times higher small for gestational age rate, without detecting additional cases at risk of stillbirth or adverse neonatal outcomes. The results suggest that the use of a twin-specific chart to monitor twin pregnancies is safe in recognizing small for gestational age–associated risk of adverse outcomes and is likely to result in fewer unnecessary investigations, interventions, and maternal anxiety.