Dichorionic twins:

• The prevalence of defects per fetus is the same as in singletons (2%) and therefore the risk of a defect in at least one fetus is 2 times as high (4%) as in a singleton pregnancy. In 10% of cases both fetuses are affected (concordance) and in 90% only one fetus is affected (discordance).

Monochorionic twins:

• The prevalence of defects per fetus is 2 times higher than in singletons and therefore the risk of a defect in at least one fetus is 4 times as high (8%) as in a singleton pregnancy. In 20% of cases both fetuses are affected (concordance) and in 80% only one fetus is affected (discordance).

Management of pregnancies discordant for abnormality:

• These pregnancies can be managed expectantly or by selective fetocide of the abnormal twin.
• In cases where the abnormality is non-lethal but may result in serious handicap the parents need to decide whether the potential burden of a handicapped child is enough to risk the loss of the normal twin from fetocide-related complications.
• In cases where the abnormality is lethal it may be best to avoid the risks associated with selective fetocide, unless the condition itself threatens the survival of the normal twin. For example, in anencephaly or trisomy 18 (with associated esophageal atresia or diaphragmatic hernia), there is >50% risk of development of polyhydramnios at 24-26 weeks’ gestation placing the normal twin at high risk of preterm birth and associated mortality and morbidity.

Selective fetocide in dichorionic twins:

• Fetocide can be carried out by intracardiac injection of potassium chloride.
• Fetocide at 11-14 weeks: risk of miscarriage 7% and risk of birth at <32 weeks 6%.
• Fetocide at ≥16 weeks: risk of miscarriage 14% and risk of birth at <32 weeks 20%.
• Fetocide at 32 weeks: this is a legal option in some countries and avoids risks of miscarriage and early preterm birth.

Selective fetocide in monochorionic twins:

• Fetocide can be carried out by occlusion of the umbilical cord vessels through endoscopic laser, ultrasound-guided bipolar forceps or radiofrequency.
• Fetocide at ≥16 weeks: risk of miscarriage 20% and risk of birth at <32 weeks 20%.