• twins
• monochorionic
• prematurity
• neurodevelopment

Twin and higher order multiples have a natural frequency of <2% of live born infants. As in vitro fertilisation (IVF) techniques have become more widespread, the frequency of multiple births has more than doubled, with the largest increase being seen in dichorionic pregnancies. Women who have IVF tend to be older and are at higher risk of associated health problems and pre-eclampsia than the general population. There is a higher rate of late fetal loss in multiple pregnancies, particularly in monochorionic pregnancies, and fetal growth is also reduced in multiple pregnancies in the third trimester. These factors lead to a high rate of preterm birth in multiple pregnancies, even when one considers that 38 weeks is generally considered as term for twin pregnancies. The high rate of preterm delivery in multiples is reflected in the workload of neonatal units, with about 30% of infants of <1.5 kg being twins or higher order multiples.1 The medical costs associated with twins in the first 5 years of life are three times that of singleton infants, with most of these costs being incurred due to neonatal admissions because of prematurity.2

Swamy et al report on the developmental outcome at school age of 51 pairs of monochorionic twins with growth discrepancy ranging from 20% to 56%. The authors are to be congratulated for a high follow-up rate of 77%. The cohort had a mean gestation of 34.7 weeks with a mean difference in birth weight of 664 g. The lighter twin had a General Conceptual Ability (GCA) score that was three points lower (small twin 105.4 vs large twin 108.4, 95% CI −0.9 to −5.0). Mathematics and memory skills showed the …