Abstract: We included one trial of 526 women with a twin pregnancy of two viable twins, with no known morphological abnormality, in this review. The trial compared women receiving fetal growth and Doppler ultrasounds at 25, 30 and 35 weeks' gestation to fetal growth alone. We judged the included study to be at low risk of bias however the risk of performance and detection bias were unclear. The primary outcome was the perinatal mortality rate (after randomisation), for which there was no evidence of a clear difference between the fetal growth + Doppler and the fetal growth alone groups (risk ratio (RR) 0.88, 95% confidence interval (CI) 0.32 to 2.41, low-quality evidence) with similar rates in both groups (seven events in the Dopper + fetal growth group and eight in the fetal growth alone group). No clear differences were seen between the two regimens for the other outcomes in this review: stillbirth (RR 0.67, 95% CI 0.11 to 3.99), neonatal death (RR 1.01, 95% CI 0.29 to 3.46, low-quality evidence), gestational age at birth (weeks) (mean difference 0.10, 95% CI -0.39 to 0.59, moderate-quality evidence), infant requiring ventilation (RR 0.86, 95% CI 0.59 to 1.25), admission to special care or intensive care units (RR 0.96, 95% CI 0.88 to 1.05), caesarean section (any) (RR 1.00, 95% CI 0.81 to 1.23, high-quality evidence), elective caesarean section (RR 1.06, 95% CI 0.77 to 1.47), emergency caesarean section (RR 0.93, 95% CI 0.66 to 1.32), induction of labour (RR 1.10, 95% CI 0.80 to 1.50, moderate-quality evidence) or antenatal hospital admission (RR 0.96, 95% CI 0.80 to 1.15, high-quality evidence). The number of preterm births before 28 weeks' gestation was not reported in the included study. For the mortality-related outcomes, event numbers were small. The included study did not report the majority of our maternal and infant secondary outcomes. Infant outcomes not reported included fetal acidosis, Apgar scores less than 7 at five minutes and preterm birth before 37 and 34 weeks' gestation. The maternal outcomes; length of antenatal hospital stay, timely diagnosis of significant complications, rate of preterm, prelabour rupture of membranes and women's level of satisfaction with their care were not reported. The study did not classify twin pregnancies according to their chorionicity. An awareness of the chorionicity may have improved applicability of this data set. We downgraded outcomes assessed using GRADE for imprecision of effect estimates. This review is based on one small study which was underpowered for detection of rare outcomes such as perinatal mortality, stillbirth and neonatal death. There is insufficient evidence from randomised controlled trials to inform best practice for fetal ultrasound surveillance regimens when caring for women with a twin pregnancy. More studies are needed to evaluate the effects of currently used ultrasound surveillance regimens in twin pregnancies. Future research could report on the important maternal and infant outcomes as listed in this review.

Plain-language summary: We searched for evidence from randomised controlled trials in August 2017. We found one multicentre study that was at low risk of bias. The study involved 526 women with healthy twin pregnancies that appeared to be normally formed on ultrasound. The women were randomised to have scans to measure either just the growth of the babies or both growth and blood flow to the babies, at 25, 30 and 35 weeks of pregnancy. Findings were similar with the different tests. The study found that having scans to measure growth alone or growth with blood flow made no clear difference to the number of babies that died in the third trimester, at birth, or in the 28 days following birth. There were also no clear differences in the number of babies who were admitted to special care units or needed help breathing (ventilation). We found no difference in numbers of women who had an elective or emergency caesarean section or who were admitted to hospital during their pregnancy (both high-quality evidence). Having different types of scans made no difference to whether women gave birth early or had to have their labour induced (moderate-quality evidence). No information was available on the diagnosis of significant complications during the pregnancy, early preterm births before 28 week's gestation, or women's level of satisfaction with their care. The study did not group the pregnancies by whether the twins shared the same outermost membrane that surrounds them, or not (their chorionicity). If they do, the two babies also share the same placenta and have an intermingled blood circulation, which increases the possibility of twin-related complications. From this one study, we still do not know if the different scanning tests and how often they are done improves outcomes for women with a twin pregnancy. Future studies could find that there are differences in the number of infant deaths with the different ways of scanning. More research is needed to find out how often scans should be done and how detailed they should be in order to see whether the twins are growing normally and to pick up any problems quickly.