Background:

• Multiple births account for 2-3% of all live births and the incidence is increasing due to assisted conception and increasing maternal age.
• Perinatal mortality and morbidity is higher with multiple pregnancies than singletons and therefore these pregnancies require additional support.
• The risk of pregnancy complications is much higher in monochorionic (MC) than dichorionic (DC) pregnancies.
• Two-thirds of twins are dizygotic (non-identical) and one-third monozygotic (identical). One-third of monozygotic twins are dichorionic (DC) and two-thirds are monochorionic (MC). Therefore, all MC twins are monozygotic and 6 of 7 DC twins are dizygotic.
• In DC pregnancies the inter-fetal membrane is composed of a central layer of chorionic tissue sandwiched between two layers of amnion, whereas in MC pregnancies there is no chorionic layer.

Determination of chorionicity:

• The best way to determine chorionicity by ultrasound at 11-13 weeks’ gestation is to examine the junction between the inter-fetal membrane and the placenta. In DC pregnancies there is a triangular placental tissue projection (λ sign) into the base of the In MC pregnancies there is no placental tissue projection into the base of the membrane (T sign).
• With advancing gestation, there is regression of the chorion laeve and the ‘lambda’ sign becomes progressively more difficult to identify. Thus by 20 weeks only 85% of DC pregnancies demonstrate the λ sign.

Pregnancy dating:

• Spontaneous conception: use the crown-rump length of the longest fetus at 11-13 weeks.
• IVF conception: use the embryonic age from fertilization.

Measurements:

• In each scan assess fetal growth (head circumference, abdominal circumference, femur length), amniotic fluid (deepest vertical pool), pulsatility index by Doppler (umbilical artery, middle cerebral artery and ductus venosus) and in monochorionic twins middle cerebral artery peak systolic velocity to detect possible twin anemia–polycythemia sequence (TAPS).
• At 20 weeks measure cervical length. If <20 mm give vaginal progesterone 400 mg in the morning and 400 mg at night and repeat the scan in 1 week. If there is progressive cervical shortening to <5 mm give prophylactic steroids for fetal lung maturity. There is no evidence that bed rest or cervical cerclage is beneficial.

Dichorionic twins:

• Scans at 12, 20 weeks and then every 4 weeks until delivery.
• If there is discordance in fetal size of >15%, discordance in amniotic fluid or any abnormal Dopplers then review every 1 week.
• If there is no complication, consider delivery at 37 weeks.

Monochorionic diamniotic twins:

• Scans at 12 and 16 weeks and then every 2 weeks until delivery.
• If there is discordance in fetal size of >15%, discordance in amniotic fluid or any abnormal Dopplers then review every 1 week.
• If there is no complication, consider delivery at 36 weeks.

Monochorionic monoamniotic twins:

• Scans at 12 and 16 weeks and then every 2 weeks until delivery.
• If there is discordance in fetal size of >15% or any abnormal Dopplers then review every 1 week.
• If there is no complication, delivery by cesarean section at 32 weeks.

Trichorionic triplets:

• 12 weeks: counsel concerning options of expectant management or embryo reduction.
• Scans at 12, 20, 24, 28 and 32 weeks.
• If there is discordance in fetal size of >15%, discordance in amniotic fluid or any abnormal Dopplers then review every 1 week.
• If there is no complication, consider delivery by cesarean section at 34 weeks.

Monochorionic or dichorionic triplets:

• 12 weeks: counsel concerning options of expectant management or embryo reduction.
• Scans at 12 and 16 weeks and then every 2 weeks until delivery.
• If there is discordance in fetal size of >15%, discordance in amniotic fluid or any abnormal Dopplers then review every 1 week.
• If there is no complication, delivery by cesarean section at 32-34 weeks.