The incidence of multiple births has risen in the last two decades due mainly to increasing use of assisted reproduction techniques and advancing maternal age. About a quarter of successful IVF procedures result in multiple pregnancies and multiple births currently account for about 2–3% of all live births. Multiple pregnancies are associated with higher risks for the mothers and babies. Preterm birth occurs in 50% of twin pregnancies, with 10% taking place before 32 weeks of gestation. Fetal risks are mainly determined by the chorionicity with twin-to-twin transfusion syndrome (TTTS) complicating monochorionic twins and accounting for 20% of multiple pregnancy stillbirths. Additional risks to the babies include intrauterine growth restriction with 60% of unexplained stillbirths being associated with a birthweight of less than the tenth centile, compared with 40% for singleton births.

This issue of Best Practice and Research Clinical Obstetrics and Gynaecology focuses on understanding, assessing and potentially mitigating these risks by using a strong evidence base. Bhattacharya and Kamath review the key evidence for maintaining success of assisted reproduction whilst minimizing multiple birth rates. The chapters relating to antenatal care begin with early pregnancy assessment which highlights the very recent data for systematic labeling of twin pregnancies on antenatal ultrasound . This chapter also reviews the optimal strategy for early pregnancy ultrasound dating, determining chorionicity and the role, if any, for first trimester fetal size discordance in assessing risk for subsequent pregnancy complications . This is followed by a series of chapters dealing with monochorionic placental inter-twin transfusion syndromes, selective fetal reduction and spontaneous fetal demise in multiple pregnancy. The three subsequent chapters deal with the two most important complications affecting twin pregnancy: preterm birth and fetal growth discordance. Although prediction of preterm birth seems feasible, unlike singleton pregnancy, most interventions appear ineffective or deleterious in multiple pregnancies . The more common problem of intertwin growth discordance at first seems easier to diagnose and manage, and Liesbeth Lewi and colleagues review the various discordance thresholds proposed for the basis of intervention .

The final three chapters deal with the general obstetric aspects of multiple pregnancy care. Leanne Bricker outlines the evidence base, or lack thereof, for the package of routine antenatal care provided ubiquitously in multiple pregnancy . There is also increasing evidence that timing of birth in multiple pregnancy needs to be dictated by chorionicity, with monochorionic twins being offered elective birth from 36 weeks’ gestation (a week earlier than dichorionic twins), after a course of antenatal corticosteroids . Finally, and most importantly, Jon Barrett extensively discusses the conduct, results and significance of the Twin Birth study on the mode of twin birth .

I wish to extend my sincere thanks to all the authors for their efforts to make the huge volume of complex scientific data digestible to readers. I am certain that this multiple pregnancy edition will make a significant contribution to the knowledge base for all specialists involved in the antenatal care of these women and their babies, and hope that it will translate into improved outcomes worldwide.