Early ultrasound scans for determining chorionicity

By Dr Emma Parry, Multiples NZ Medical Advisor

There is an increased maternal and fetal risk in twin pregnancies, more-so if it involves monochorionic fetuses. Chorionicity (how the membranes are set up) is determined by ultrasound—and the earlier the better, and definitely before 16 weeks! After this time, the babies are too big to accurately determine the chorionicity.

If an ultrasound is performed early enough in the pregnancy, the yolk sac (which disappears gradually as an embryo develops) may be visible on the scan. It is worth trying to find out if there is one sac or two, as some research suggests that there is a direct relation between the number of amniotic and yolk sacs.

Generally, if there are two yolk sacs, then there will be two amniotic sacs. If a woman has been diagnosed as carrying monoamniotic twins, but saw two yolk sacs on an early scan, then there is the possibility that she has been misdiagnosed. It is reason enough to continue looking for a membrane dividing the embryos, unless and until she has separate confirmation of the diagnosis.

The amniotic membrane can be very thin... so thin, that it's extremely hard to see on an ultrasound, making misdiagnoses common. If the diagnosis was made before 8 weeks, it is definitely not certain: it is simply not possible to get a reliable determination of whether a membrane is present or not before 8 weeks. The best time to check for the membrane is between 10 and 12 weeks, and again between 16 and 20 weeks.

At these times the membrane is usually easier to see. If you are diagnosed before that time, you may want to ask for a repeat ultrasound during one or both of these time-frames. Once you are referred to an Obstetrician, you are likely to have fortnightly ultrasounds, which should ensure you have at least one scan during these times.

If you only discover that you’re carrying twins later in the pregnancy and the sonographers can’t tell whether there is a membrane present, there is still a good chance that the membrane was hidden at the time of the scan.

Finally, if an ultrasound reveals the presence of tangles in the cords of both babies, that is fairly determinative of the babies sharing a single sac. Even if there is a membrane, because it didn't prevent the cords from becoming entangled, it is safest to manage the pregnancy as if the babies are monoamniotic.

The first scan, if done prior to 15 weeks, will have nearly 100% accuracy in determining chorionicity. It is standard practice for sonographers to comment on chorionicity at the time of an early scan. If a scan at less than 15 weeks confirms a twin pregnancy but does not clearly state chorionicity, it should be repeated or reviewed.

Nuchal translucency is as accurate in twin pregnancies as in singletons and can   provide individual risk of aneuploidy (a type of chromosome abnormality) for each fetus. Any discordancy in nuchal translucency, however, can also be due to an increased risk of development of Twin to Twin Transfusion Syndrome (40% PPV – positive predictive value) or due to other structural abnormality.

If you are diagnosed with carrying monochorionic (MC) twins, whether diamniotic or monoamniotic, you will be classed as having a high-risk pregnancy. In these circumstances you should be managed by either a specialist LMC, or with regular obstetric specialist input, and can expect to undergo regular ultrasound scans.

The following scans are recommended for MC twins:

• Dating scan (with determination of chorionicity)
• Nuchal translucency scans
• Fortnightly scans from 16 weeks gestation for growth and liquor to detect early signs of TTTS.
• Where any of these scans indicate any of the following situations, a specialist with an interest in high-risk pregnancy should be involved:
• Discordant nuchal translucency
• Liquor discordancy
• Growth discordancy of greater than 20% EFW (estimated fetal weight).