Discordant Twin Growth

Discordant twin growth is a difference in the estimated weights of twins, with mild discordance at 15 to 20 percent, moderate at 20 to 25 percent and severe at above 25 percent. It reflects unequal placental share, abnormal cord insertion, chromosomal or structural disease in one twin, infection, or vascular connections in monochorionic twins. It matters because the smaller twin has a higher risk of death, and the measure guides how often to scan and when to deliver.

• Mild discordance up to 20 percent is common and usually benign.
• Risk factors: monochorionic placentation, velamentous cord insertion, maternal hypertension, smoking.
• Severe discordance carries a higher perinatal mortality.

Women have no symptoms of discordance. The finding is on ultrasound, or from a fundal height that lags or rises abnormally, with reduced movements of one fetus in some.

• Fundal height may be below expected for twins.
• No maternal signs.
• Differential diagnosis: twin to twin transfusion syndrome, selective growth restriction, twin anaemia polycythaemia sequence, wrong dates, measurement error, single fetal anomaly.
• Complications: death of the smaller twin, preterm birth, preeclampsia, brain injury, caesarean section, neonatal hypoglycaemia.

The approach is correct measurement and classification by percentage and chorionicity.

• Estimated fetal weight of each twin at one visit (Hadlock formula using head circumference, abdominal circumference and femur length); percentage = (larger minus smaller) divided by larger, times 100.
• Umbilical artery Doppler, middle cerebral artery peak systolic velocity, deepest vertical pocket in each sac, fetal bladders.
• Chorionicity from the first trimester scan; cord insertions; anomaly scan of each twin.
• Maternal blood pressure, urine protein, full blood count; TORCH serology and karyotype if an anomaly is found.

The principle is to scan more often as the percentage rises, to find a monochorionic cause and to deliver on time.

• Supportive: scans every 4 weeks for discordance below 20 percent, every 2 weeks for 20 to 25 percent and every 1 week for above 25 percent or abnormal Doppler; daily fetal movement counts; birth in a unit with neonatal intensive care.
• Pharmacological: aspirin 150 mg oral at night from 12 weeks to 36 weeks; ferrous sulphate 200 mg oral 8-hourly for anaemia; betamethasone 12 mg IM twice, 24 hours apart, between 24 and 34 weeks and 6 days if delivery is likely; magnesium sulphate 4 g IV over 20 minutes then 1 g per hour until delivery for 24 hours at most before 32 weeks; oxytocin 10 IU IM at birth.
• Procedures: delivery at 37 weeks for dichorionic twins with discordance below 25 percent and normal Doppler, at 34 to 36 weeks for severe discordance, and at 36 weeks for monochorionic twins, earlier for abnormal Doppler or cardiotocography; vaginal birth if the first twin is cephalic with continuous cardiotocography; caesarean section for abnormal tracing.
• Special groups: monochorionic twins with discordance of 25 percent or more or an abnormal Doppler need referral to a fetal medicine unit.
• Red flags: absent or reversed end diastolic flow, reduced fetal movements, abnormal cardiotocography, rapid increase in discordance.
• Follow-up: growth and Doppler scans at the interval set by the percentage; newborn weight, glucose and temperature checks; paediatric review for the smaller twin.