Multiple pregnancies are increasing due to advance maternal age at delivery and use of artificial reproductive techniques. Presence of two fetuses is termed twin pregnancy. Presence of three or more fetuses in the womb is known as higher order multiple pregnancy.
Multiple pregnancies are associated with increased risk of medical complications like diabetes mellitus, hypertension to the mother. Babies are at risk of abortion ,low birth weight, preterm birth and complications related to prematurity.
Multiple pregnancies can be discordant for growth (one fetus is growth restricted and the other growing normally) or discordant for anomaly (one fetus has structural abnormality and the other fetus is structurally normal.)
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Twin pregnancies are of three types.
Dichorionic diamniotic twin pregnancy (more common)
In this type of pregnancy there are two placentae (one placenta each) and two amniotic sacs. These twins are like two different individuals staying in their own room with their own kitchen.
Monochorionic diamniotic twin pregnancy
In this type of pregnancy, there is one placenta and two amniotic sacs. These twins are like two individuals staying in two rooms with a single kitchen
Monochorionic monoamniotic twin pregnancy (rare)
In this type or pregnancy, there is one placenta and and one amniotic sac. Twins in this type share both the placenta and amniotic sac.
Complications unique to monochorionic pregnancies:
In monochorionic pregnancies, umbilical cord of two fetuses join the same placenta. Blood vessels from both fetuses anastomose within the placenta and the blood circulation of both fetuses is connected with each other . Any imbalance in the blood flow within the placenta will lead to life threatening complications like selective fetal growth restriction, twin to twin transfusion syndrome, twin anemia polycythemia sequence, twin oligo polyhydramnios sequence, twin reversed arterial perfusion and conjoined twins.
Monochorionic twins are at a risk of sudden intrauterine death. In addition to the above mentioned complications, monochorionic monoamniotic twin pregnancies have higher perinatal mortality due to cord entanglement.
Management options depend on the type of complication detected.
Importance of determining Chorionicity early in pregnancy:
Chorionicity in multiple pregnancies can be determined by ultrasound as early as 6 weeks of gestation. Amnionicity can be determined by 9 weeks of gestation by ultrasound.
It is essential to label twins as dichorionic twins or monochorionic twins latest by 11-13wks for the following reasons:
- Perinatal outcome depends mainly on Chorionicity. Dichorionic pregnancies have a better outcome when compared to monochorionic pregnancies.
- Screening for aneuploidy: In the first trimester screening test for aneuploidy, methods used to calculate risk for aneuploidy are different for dichorionic and monochorionic twin pregnancies.
- Monitoring the pregnancy: Monochorionic pregnancies need to be monitored more frequently when compared to dichorionic twin pregnancies to identify complications unique to monochorionicity early in gestation and treat appropriately. Uncomplicated monochorionic pregnancies are monitored by ultrasound every 2 weeks and dichorionic pregnancies every 4 weeks.
- Management of twins discordant for anomaly: Fetal reduction by injecting KCL into the abnormal fetus can be offered in dichorionic twins discordant for anomaly. Whereas in monochorionic twins discordant for anomaly, fetal reduction methods include bipolar cord occlusion or radiofrequency ablation.
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