Management of Monochorionic Twin Pregnancy Green‐Top Guideline No. 51 (2024 Partial Update)

| Diagnosis of Monochorionic Twin Pregnancy1.1.1| How Is Monochorionicity Diagnosed Prenatally and What Is the Accuracy of Prenatal Ultrasound Chorionicity Allocation?All women with a twin pregnancy should be offered an ultrasound examination between 11 +2 and 14 +1 weeks of gestation (crown-rump length 45-84 mm) to assess fetal viability, gestational age (i.e., dating of the pregnancy) and allocate chorionicity.It will also exclude major congenital malformations.[B] Chorionicity should be determined at the time the twin pregnancy is detected/diagnosed by ultrasound.This is based upon the number of placental masses, the appearance of the intertwin membrane attachment to the placenta and the membrane thickness.This scan is best performed before 14 weeks of gestation.After this gestation, fetal sex determination may also help with the allocation of chorionicity.[D] Updated 2024 An electronic ultrasound image record should be taken documenting the ultrasound appearance of the membrane attachment to the placenta and linked to the individualised patient records (Appendix 2).[GPP] Updated 2024 If there is uncertainty about the diagnosis of chorionicity, a photographic record of the ultrasound appearance of the membrane attachment to the placenta should be retained and a second opinion should be sought.[GPP] If there is still doubt in the diagnosis of chorionicity, the woman should be referred to a fetal medicine specialist without delay, as chorionicity is best determined before 14 weeks of gestation.[D] On ultrasound, the fetuses in twin pregnancies should be assigned nomenclature (i.e., upper and lower, or left and right) and this should be clearly documented in the woman's case notes to ensure consistency throughout pregnancy.Fetus A should be with amniotic sac closest to the maternal cervix.[C] 1.2 | Outcome of Monochorionic Twin Pregnancy 1.2.1 | What Is the Outcome of Monochorionic Compared With Dichorionic Twin Pregnancies?Clinicians and women should be aware that monochorionic twin pregnancies have higher fetal loss (miscarriage, stillbirth, and neonatal death) rates than dichorionic twin pregnancies, and overall may have a higher risk of associated neurodevelopmental morbidity.This should form part of the parental counselling.[GPP]