Selective Growth Restriction in Twins

Selective growth restriction in twins is a pregnancy in which one twin has an estimated fetal weight below the 10th centile, with a weight difference of 25 percent or more between the twins. It results from unequal sharing of the placenta, and in monochorionic twins from vascular connections that complicate the picture. It matters because it carries a high risk of death of the smaller twin and of brain injury in the larger one.

• Monochorionic types by umbilical artery Doppler in the small twin: type I (positive diastolic flow), type II (persistent absent or reversed flow), type III (intermittent absent or reversed flow).
• Risk factors: velamentous or marginal cord insertion, monochorionic placentation, maternal hypertension, smoking.
• Type III carries risk of sudden death of the small twin and injury to the co-twin.

Women are usually asymptomatic. The finding is on ultrasound, and some report reduced movements of the smaller twin. The uterus may be smaller than expected for twins.

• No maternal signs in most.
• Hypertension in some.
• Differential diagnosis: twin to twin transfusion syndrome, twin anaemia polycythaemia sequence, discordant growth without growth restriction, fetal anomaly, infection, wrong dates.
• Complications: death of the smaller twin, acute transfusion causing injury or death of the co-twin, preterm birth, cerebral palsy, preeclampsia, stillbirth, neonatal death.

The approach is ultrasound with Doppler and classification by chorionicity and flow.

• Estimated fetal weight of each twin below the 10th centile for the smaller twin and discordance 25 percent or more (monochorionic) or 20 percent or more (dichorionic).
• Umbilical artery Doppler of the small twin: type I (forward end diastolic flow), type II (persistent absent or reversed flow), type III (intermittent absent or reversed flow); middle cerebral artery Doppler of both twins; ductus venosus Doppler of the small twin.
• Deepest vertical pocket in each sac; cord insertion sites; fetal anomaly scan; fetal echocardiography.
• Maternal blood pressure, urine protein, TORCH serology, antiphospholipid antibodies; karyotype of the small twin if an anomaly is found.

The principle is surveillance with fetal medicine referral and delivery timed by the Doppler type, or intervention in severe disease.

• Supportive: admit after 26 weeks for absent or reversed end diastolic flow; cardiotocography once or twice daily; daily fetal movement counts; counselling on the risks to both twins; birth in a unit with neonatal intensive care.
• Pharmacological: aspirin 150 mg oral at night from 12 weeks to 36 weeks; betamethasone 12 mg IM twice, 24 hours apart, between 24 and 34 weeks and 6 days before planned delivery; magnesium sulphate 4 g IV over 20 minutes then 1 g per hour until delivery for 24 hours at most before 32 weeks; labetalol 200 mg oral every 8 to 12 hours (maximum 2.4 g daily) or nifedipine modified release 30 to 60 mg oral once daily (maximum 120 mg) for blood pressure 140/90 mmHg or more; oxytocin 10 IU IM at birth.
• Procedures: delivery by caesarean section at 34 to 36 weeks for type I monochorionic, 32 to 34 weeks for type II, 30 to 32 weeks for type III, and at once for abnormal ductus venosus or tracing; fetoscopic laser coagulation or radiofrequency ablation or bipolar cord coagulation of the small twin in a fetal medicine unit at 16 to 26 weeks for severe type II or III disease; dichorionic twins: delivery at 34 to 37 weeks by the Doppler of the small twin.
• Red flags: absent or reversed end diastolic flow, abnormal ductus venosus, fetal death of one twin, abnormal cardiotocography, reduced fetal movements, preeclampsia.
• Follow-up: Doppler once or twice weekly by type; growth scans every 2 weeks; newborn glucose and haemoglobin; cranial ultrasound of the co-twin after birth if there was a loss.