Prenatal screening in multiple pregnancies – twin and triplet pregnancies and genetic testing

Women who discover they carry a multiple pregnancy (twins, triplets) might naturally become concerned about having to undergo more genetic testing than usual.

The truth is that so long as there are no specific risks for genetic disorders, multiple pregnancies do not increase the risk of hereditary genetic diseases. However, it is important to consider that birth defects are still more prevalent in multiple pregnancies. 


Why is a multiple pregnancy considered high-risk?

As mentioned, twin pregnancies are not associated with an increased risk of genetic diseases but do involve an increased risk of birth defects and are therefore defined as high-risk pregnancies.

  • Pregnancies with only one baby also involve a certain risk of development defects due to blood circulation disruptions, but in multiple pregnancies this risk is especially high. This may be caused by certain factors such as a connection between the placentas, one fetus applying pressure on blood vessels leading blood to the other fetus, in cases of blood clots transferred from one fetus’ placenta to the other, or twin-to-twin transfusions.
  • Twin pregnancies involve a higher risk of intellectual disability. In pregnancies with only one baby, this risk is around 3-4%, compared to a risk of around 6-8% in twin pregnancies.
  • The general risk for diseases that develop during the pregnancy is doubled in twin pregnancies and tripled in triplet pregnancies.
  • In some cases, one of the twins can apply physical pressure on their sibling, which can result in physical defects in the organ under pressure.
  • The risk of premature birth and birth complications is higher in multiple pregnancies.

Considering these factors, multiple pregnancies may not involve a higher risk of genetic disorders but do involve a higher risk of birth defects that are not genetically related.


Chromosomal testing

When comparing the level of importance of undergoing invasive chromosomal screening (such as amniocentesis) in a singleton pregnancy and the level of importance of undergoing such tests in a multiple pregnancy, we can see that as long as there are no relevant risk factors, the level of importance is the same. Moreover, amniocentesis and CVS are procedures that come with a certain risk of miscarriage.

What many people are unaware of is that the risk of miscarriages after invasive testing in a multiple pregnancy is higher than the risk of a miscarriage in a singleton pregnancy (more than twice the risk). 

Undergoing invasive chromosomal testing should only be chosen when there is clear medical evidence that one of the fetuses is in risk of chromosome abnormalities.


Alpha-fetoprotein testing

In a multiple pregnancy, it may become more difficult to assess the risk of Down syndrome via alpha-fetoprotein tests, that are performed using blood tests. In some cases, one fetus can develop the disorder, but the healthy fetus can “cover” for these findings, which can result in an incorrect diagnosis.


Nuchal translucency test

The test of choice for diagnosing Down syndrome in multiple pregnancies is a nuchal translucency test. This is because in nuchal translucency testing, each fetus is tested separately (unlike alpha-fetoprotein tests).

It should be noted that undergoing nuchal translucency tests does not dismiss the importance of undergoing alpha-fetoprotein tests in weeks 16-18 of pregnancy, since alpha-fetoprotein tests allow detecting neural canal defects.


What to do if one of the babies is diagnosed with a severe disease or birth defect?

If one of the fetuses is diagnosed with a severe disease or defect, parents can choose to undergo a selective reduction, according to the medical condition. Selective reduction is a procedure for reducing the number of fetuses in a pregnancy. In almost all cases, selective reduction does not harm the other healthy developing fetuses.


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