Case definition
All MC twin pregnancies with a sIUFD after 14 weeks gestation, including spontaneous IUFD (i.e. no signs of TTTS, sIUGR or congenital/structural anomaly), IUFD after intervention for complications such as fetoscopic laser ablation (FLA) for TTTS, sIUGR or selective feticide (intrafetal laser, radiofrequency ablation and cord occlusion) were included. Those with “major” structural fetal anomalies (concordant and discordant) were included, as were MC monoamniotic twins. For inclusion, chorionicity had to have been confirmed in the first trimester based on the presence of the ’T’ sign and absence of the ‘lambda’ sign, and a single placental mass13-15. The following were excluded: higher order multiples where fetal reduction had been performed, pregnancies with twin reversed arterial perfusion sequence. TTTS was defined according to the Quintero definition16, and sIUGR as an inter-twin growth discordance greater than 20% in estimated fetal weights as at the time of designing the study there were no validated twin-specific growth charts.