Protocol for ultrasound scan to detect Multiple Pregnancy: Dr Vidhya Moorthy

An update to care and management of multiple pregnancy- Dr Vidhya Moorthy

The incidence of multiple pregnancy is 9 per 1000 live births in India. This is increasing in the last few decades with increasing rates of assisted conceptions with more than one embryo transfer. Hence it is important for clinicians to remain up to date with the management of these high-risk pregnancies. Ultrasound scan plays a vital role in the diagnosis of the multiple pregnancy, to determine the chorionicity and amnionicity, to formulate a plan of care for the rest of the pregnancy based on that.

Indizygotic twins each fetus has its own placenta and amniotic sac. In monozygotic pregnancies, there may be a sharing of the same placenta (monochorionic), amniotic sac (monoamniotic) or even fetal organs (conjoined). One-third of monozygotic twins are dichorionic (DC) and two-thirds are monochorionic (MC). Therefore, all MC twins are monozygotic and 6 out of 7 DC twins are dizygotic. In case of monochorionic twin pregnancy more frequent scans to assess for selective fetal growth restriction and twin to twin transfusion syndrome is essential. Hence evidence-based protocols for ultrasound scan in multiple pregnancy is mandatory in every high-risk pregnancy unit, especially the fertility centres.

Viability scan:

• The aim of the ultrasound scan is to confirm intra-uterine pregnancy, demonstrate the fetal heartbeat and to identify the number of fetuses. The inter-fetal membrane is very thin to visualize at this early gestation and chorionicity and amnionicity is best determined at 11-14 weeks.

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.

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.

Identify the fetus:

• It is important to identify the fetuses and name them as A, B, C and to clearly document it. The fetus closer to the cervix is named usually as A and the position of the other fetus/ fetuses should be named in relation to the fetus A (upper/ lower, right/ left) This is important for invasive procedures and selective fetal reduction. The varying position of the placenta and the gender can be helpful to identify the fetus in the later gestation.

First trimester aneuploidy screening:

• Women who wish to have aneuploidy screening should be offered nuchal translucency measurements in conjunction with first trimester serum markers (combined screeningtest) at 11 weeks to 14 weeks of gestation (crown–rump length 45–84 mm).


• In Dichorionic twins, the risk is calculated for each fetus and the NT measurements of each fetus is used. In monochorionic pregnancy, the risk is calculated for the pregnancy and the average of the two NT measurement has to be used.

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 venous) 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 <25 mm, vaginal progesterone administration is recommended as per local protocol. If there is progressive cervical shortening then give prophylactic steroids for fetal lung maturity. There is no good evidence to support bed rest or cervical cerclage.

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 Doppler 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 Doppler 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 caesarean 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 Doppler then review every 1 week.


• If there is no complication, consider delivery by caesarean 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 Doppler then review every 1 week.


• If there is no complication, delivery by caesarean section at 32-34 week