Multiple Pregnancy Scans

What is the difference between non-identical and identical twin/ multiple pregnancy?

Fertilization of egg by a sperm is called zygote which later develops into embryo and then baby.

In non-identical twins (DIZYGOTIC), each egg is fertilized by different sperm. It is like two/ multiple babies with different genetic material growing in the womb at the same time.

This usually happens two or more eggs ovulate at the same time and get fertilized or two or more embryos were transferred in intro-fertilization process.

Identical twins (MONOZYGOTIC) develop from one egg and one sperm forming one zygote. Very early in the process, the fertilized egg or zygote splits into two or more embryos.

This is the concept of ZYGOSITY.

What are the type of twin pregnancies?

Monozygotic are the identical twins. They can be

• Dichorionic Diamniotic- when each baby has their own placenta and sacs.
• Monochorionic Diamniotic- When there is single placenta, the placenta is shared among the twins and each twin has their own sac.
• Monochorionic Monoamniotic- When the twin shares the placenta as well as the sac- single placenta and single sac.

Dizygotic will always be dichorionic and diamniotic- each baby has their own placenta and sacs.

This the concept of CHORIONICITY.

What is chorionicity?

Chorionicity is to determine the type of placentation. Hence, we determine chorionicity and zygosity on the ultrasound.

Determination of chorionicity is important as it determines the type of complications the babies can have and the type of follow up they require.

What are the ultrasounds that happen in the multiple pregnancy?

• Early viability scan- this scan is to determine the number of fetuses and type of multiple pregnancy, that is to determine the CHORIONICITY.
• First trimester screening- at 11-13+6weeks- Here, we determine the number of fetuses, type of multiple pregnancy, dating, structural abnormalities and screen for chromosomal abnormalities.
• If it is monochorionic diamniotic pregnancy, then screening for specific complications like twin to twin transfusion syndrome where one fetus bleeds into the other or selective weak baby starts from 16 week onwards. Then the scans are done every 2 weekly till 24 weeks.
• Anomaly scan -done between 18-20 weeks- to do detailed evaluation of the structural abnormalities of the babies.
• Growth and Doppler assessment – to assess fetal weight, fluid around the baby and their respective blood flows. It is done every 4 weekly after anomaly scan in dichorionic diamniotic pregnancies and every 2-3 weekly in monochorionic pregnancies.

Does every multiple pregnancy require the same protocol of monitoring?

The care has to be individualised for every multiple pregnancy. As these pregnancies are at risk of early delivery, weak babies, developing high blood pressure and sugars, hence the monitoring of babies by ultrasound needs to be individualised.

Also, the monochorionic pregnancies have unique set of complications like twin to twin transfusion syndrome (TTTS) where one baby bleedss into the other twin or selective fetal growth restriction, where one baby grows really small in comparison to the other baby. Hence, depending on how each multiple pregnancy behaves, we have to alter the protocol.

I can understand that these are somewhat difficult to understand but if you have any queries, I would be eager to answer all of them.