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Before choosing any screening test in a twin pregnancy, your doctor needs to answer two questions. Are the twins identical or non-identical? And do they share one placenta, or does each baby have its own? The answers shape almost every screening decision that follows.
Identical and non-identical twins
Identical twins come from one fertilized egg that splits into two. They share the same DNA. If one baby has a chromosome condition such as Down syndrome, the other baby almost always has it too.
Non-identical twins come from two separate eggs fertilized by two separate sperm. Genetically, they are like any two brothers or sisters. One baby can have a chromosome condition while the other does not.One placenta or two
Doctors use the word “chorionicity” to describe how many placentas the twins have.
Twins sharing one placenta (monochorionic twins) are almost always identical.
Twins with two placentas (dichorionic twins) are usually non-identical, but not always. About one in three identical twin pairs also have two separate placentas. This happens when the embryo splits very early, in the first few days after fertilization. So “two placentas” does not prove your twins are non-identical.How doctors find out
The number of placentas is best seen on an ultrasound scan in the first trimester, ideally before 14 weeks. After that, it becomes much harder to tell. This makes the early scan one of the most important scans of a twin pregnancy. Ask your doctor to note the number of placentas clearly in your scan report.
Why it changes screening
How results are read: With twins sharing a placenta, a chromosome result usually applies to both babies. With non-identical twins, the result may apply to one baby, both, or neither, and a blood test cannot tell which.
The double marker test: This blood test measures two substances made by the placenta. In twins, both placentas release these substances into the same bloodstream, so a healthy baby’s values can mask an affected baby’s values. The number of placentas also affects these measurements, so the risk must be calculated with software adjusted for twins.
Nuchal translucency: This measurement of fluid at the back of each baby’s neck, taken between 11 weeks and 13 weeks 6 days, gives information about each baby separately. In twins, it carries more weight than the blood markers.
Diagnostic testing: If a result is concerning, twins with two placentas usually need each baby tested separately. For twins sharing a placenta, one sample is often enough when both babies look alike on scan.Extra monitoring for twins sharing a placenta
Twins sharing a placenta need frequent scans, often every two weeks from around 16 weeks. These look for complications such as twin-to-twin transfusion syndrome, where blood flow between the babies becomes unbalanced. In these twins, a large difference in neck fluid or size between the two babies can also be an early sign of placental complications, not only of a genetic condition.
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