Hidden high rate of pre-eclampsia in twin compared with singleton pregnancy.
To examine the gestational age at delivery in dichorionic (DC) and monochorionic (MC) twin pregnancies, with and without pre-eclampsia (PE), and to determine the relative risk of total and preterm PE compared with that in singleton pregnancies. This was a screening study for PE in twin pregnancies undergoing first-trimester combined screening for aneuploidy and subsequently delivering two phenotypically normal live or stillborn babies at ≥ 24 weeks' gestation. The distribution of gestational age at delivery in DC and MC twins was determined and compared with that in singleton pregnancies from the same population. The relative risk for total and preterm PE in twins compared with singleton pregnancies was determined. Kaplan-Meier estimates of the cumulative incidence of PE in twin and singleton pregnancies, assuming no other cause for delivery, were determined and hazard ratios for twins relative to singletons were obtained from a Cox proportional hazards regression model. The incidence of PE in singletons was 2.3% (2162/93 297), in DC twin pregnancies was 8.1% (145/1789) and in MC twin pregnancies was 6.0% (26/430). Compared with singletons, the relative risk of total PE was 3.5 for DC twins and 2.6 for MC twins. Delivery < 37 weeks' gestation occurred in 5.5% of singletons, 46.5% of DC twins and 91.4% of MC twins. The incidence of preterm PE was 0.6%, 5.5% and 5.8% for singletons, DC twins and MC twins, respectively. Compared with singletons, the relative risk of preterm PE was 8.7 for DC twins and 9.1 for MC twins. In the Cox proportional hazards regression model, the hazard ratios for DC and MC twin pregnancies relative to singleton pregnancies were 14 and 23, respectively. The relative risk of preterm PE in DC and MC twins is similar and substantially higher than in singleton pregnancies. In ongoing twin pregnancies, the high relative risk of PE may merit a higher intensity of monitoring than is routine for singleton pregnancies. © 2017 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of the International Society of Ultrasound in Obstetrics and Gynecology.