What are USG OBST NT TWINS ?
Yes, you can have a nuchal translucency (NT) scan if you’re expecting twins. In fact, if you’re pregnant with twins, NT is the best screening method to test for Down’s syndrome. The scan measures the size of a fold behind your babies' necks.
The NT scan will be combined with a blood test that checks your levels of two hormones, hCG and PAPP-A. The scan plus the blood test is called the combined test, and it estimates the chance that one or both of your babies has Down's syndrome.
The combined test is done in the first trimester, between 11 weeks plus two days and 13 weeks plus six days of your pregnancy.
You will also be offered a chorionicity scan at the same time. This scan finds out if your twins share a placenta or not.
- If your twins don’t share a placenta, they are non-identical. In other words, they don’t share the same genetic setup. So, the individual risk of Down’s syndrome can be known for each baby. It may happen that one of your twins has a low risk, and the other has a high risk.
- If your twins share a placenta, they are identical. This means, either both will have Down’s syndrome, or both will be unaffected. In this case, the risk will be given for the pregnancy as a whole, instead of the risk for each baby.
If you miss the chance to have the combined test before 14 weeks, your doctor may still offer a test in the second trimester. This test is called the quadruple test and is done between 15 and 20 weeks of pregnancy.
The quadruple test measures four proteins, or markers, in your blood. These are:
If you're carrying a baby with Down's syndrome, you will have higher levels of hCG and inhibin A in your blood. You will have lower levels of AFP and uE3.
However, many doctors don't do a quadruple test when the mum is expecting twins, because:
- Without the information gained from the NT scan, a blood test at this stage isn't as accurate in twin pregnancies. It is not recommended if you are expecting triplets.
- The quadruple test works on the fact that babies with Down’s have different levels of the four markers. However, say, for example, one of your twins has Down’s and the other doesn’t, the normal levels of AFP of one baby will show up. Whereas, the low levels of AFP of the other baby will not show up.
Non-Invasive Prenatal Testing (NIPT) can be done for twins. NIPT is more accurate and can be done earlier in your pregnancy (from about 9 to 10 weeks). It also carries no risk of miscarriage.
A normal or negative test suggests that both babies are normal. A positive test suggests there is a problem, but it cannot distinguish which of the babies has a chromosomal defect. In such circumstances, amniocentesis will be necessary for a final conclusion. NIPT cannot be performed for triplets or more.
If the results of the tests show you are at high risk of carrying one or two babies with Down's, to get a definitive diagnosis, your doctor might recommend that you have a chorionic villus sampling (CVS) or an amniocentesis.
However, there is a risk of miscarriage after a CVS or amniocentesis. The risk is about one in 100 in a single pregnancy and double that in a twin pregnancy.
It might be very distressing to find out that you have one or two babies with a high risk of Down's. In such a case, you may have to make a difficult choice about your next steps. Your doctor will be there to support you along the way.
