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Before 14 weeks’ gestation, ultrasound is 99% sensitive and 100% specific for chorionicity, and it’s an easy assessment to do. In the second trimester, it gets trickier, which is why determining chorionicity early is so important, he said.
Dr. Rand gave several recommendations for management of monochorionic twins. In the first trimester, he said, determine the chorionicity, check the nuchal translucency, and assess fetal growth.
In the second trimester, conduct level II (high risk) anatomical surveys and echocardiograms to screen for fetal cardiac disease (as you would in a diabetic mother). Establish a minimum surveillance schedule of at least every 2 weeks for amniotic fluid between 16 and 28 weeks’ gestation, and every 4 weeks for fetal growth between 16 and 32 weeks’ gestation.
In the third trimester, continue the surveillance of amniotic fluid and fetal growth. Consider performing non–stress tests and Doppler ultrasounds. There’s debate about the optimal gestational age for delivery of monochorionic twins, between 36 and 38 weeks. "We’re not sure when to deliver," Dr. Rand said, but he recommends delivering monochorionic twins who’ve had issues with growth, fluid, or other aspects during pregnancy closer to 36 weeks and delivering twins with no issues closer to 38 weeks. "Monochorionicity is not, in and of itself, an indication for cesarean delivery," he added.
Approximately 75% of twins are dizygotic (from two separate eggs, each fertilized by sperm) and thus dichorionic. Among the 25% of twin pregnancies that are monozygotic (from one egg), the egg splits more slowly in 75% of cases, creating monochorionic twins that share a placenta and circulation. In the other 25% of monozygotic twins, the egg splits quickly, within 3 days, yielding two separate units and creating dichorionic twins, each with their own placenta.
Dr. Rand reported having no financial disclosures.
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