Most couples want to know if their pregnancy is at high risk for a fetal chromosomal problem such as Down syndrome. Working out this risk is referred to as “prenatal screening.” There are some important differences when considering prenatal screening in a multiple pregnancy. Screening in multiple pregnancies should be overseen by an experienced Maternal-Fetal Medicine specialist, who can provide the full range of testing options, including invasive testing if required.
Monozygotic twins are genetically identical – therefore either both twins will have a chromosomal problem or neither will. Dizygotic twins are genetically non-identical – so a problem might affect one or both twins.
What tests are available?
As with singleton pregnancies, there are 2 main screening options for women expecting twins:
- Nuchal Translucency (NT) screening
- Non-Invasive Prenatal Testing (NIPT) (e.g. GenesyteTM, HarmonyTM, PanoramaTM tests)
Traditionally, NT screening was preferred in twin pregnancies although more recent studies have confirmed that NIPT performs well with twins. However, NIPT is not suitable for women with triplets or higher-order multiples. In these cases, only nuchal translucency measurement (without biochemistry) may be offered.
1. Nuchal translucency (NT) screening in multiple pregnancy
- This test must be performed between 11+0 and 13+6 weeks. The test requires a blood test and a specialised ultrasound scan. The blood test analyses 3 pregnancy proteins (hCG, PAPP-A and placental growth factor) and the scan measures a film of fluid at the back of the fetal neck, called the “nuchal translucency”. An individual measurement is taken for each fetus.
- We usually ask women to have their blood taken at the pathology centre several days prior to the ultrasound appointment. This way, the full screening result can be discussed with couples immediately after the ultrasound scan. It is very important that the woman’s due date is accurate, so that the blood test can be interpreted correctly by the laboratory. It is also very important that the laboratory is made aware that this is a multiple pregnancy.
- The ultrasound scan is crucial and should be undertaken by a fetal medicine specialist who is certified in performing NT scanning.
- NT screening gives the probability of a baby having Down syndrome, Edwards syndrome and Patau syndrome. Because the NT is measured separately for each fetus, an individual risk (a 1-in-X number) can be calculated for each baby. With monochorionic twins (genetically-identical), the average of the 2 risks is taken as the overall “pregnancy risk”.
2. Non-Invasive Prenatal Testing (NIPT) in multiple pregnancy
- This newer screening test has been available internationally for the past 3-4 years and is now widely used in Australia. The test simply involves a maternal blood test, which can be taken any time after 10 weeks. The mother’s blood contains small quantities of fetal DNA, which is extracted in the laboratory and tested for the 3 common chromosomal disorders.
- When first introduced, the NIPT test was not validated for use in twins. However recent studies have confirmed that NIPT testing performs well in twin pregnancies. It is not currently available for triplets or higher-order multiples.
- Although the NIPT test can also determine the fetal gender, interpretation is more complex in twin pregnancies. The test can only determine whether there is DNA from the Y chromosome present or not. For monochorionic twins, this means that both twins are boys. For dichorionic twins, this means that at least one twin is a boy. Absence of a Y chromosome in both monochorionic and dichorionic twins means two girls.
- NIPT testing should be used with caution in cases of a “vanishing twin” (where 1 twin stops developing early in the pregnancy, often resulting in an extra empty sac on scan). This is because DNA from the vanishing twin could contaminate the result and lead to a false positive result.