Prenatal Testing

In the past, women often weren’t sure if they were having Twins until they actually gave birth. Twins were often considered a “surprise,” to be shared in the delivery room, along with the gender reveal. Today, however, a double pregnancy is often identified fairly quickly, sometimes as soon as the first routine ultrasound.

Prenatal Testing Different with Twins

Finding out that you’re pregnant with twins can be a joyous occasion that also brings its own specific set of questions and concerns. How is a pregnancy with twins different than pregnancy with a single baby? How will prenatal care differ? How will you know if the babies are healthy? How does non-invasive prenatal testing differ when there are multiple babies?

Non-Invasive Prenatal Testing

Non-invasive prenatal testing (NIPT) can be performed within the first trimester of pregnancy to check the fetuses for many known genetic or chromosomal abnormalities (like cystic fibrosis or Down syndrome). Testing is safe for the fetuses because it only requires a blood draw from the mother to collect free-floating fetal DNA in the mother’s bloodstream.

Monozygotic and Dizygotic Twins

The accuracy of NIPT in multiple births can vary depending on whether or not the mother is pregnant with monozygotic or dizygotic twins. It’s important that doctors determine what kind of twins a woman is having before beginning NIPT and before analyzing NIPT results. Monozygotic twins are often called “identical” twins and are the product of one zygote (one sperm and egg pairing) that divides to form two embryos. Dizygotic twins are often referred to as “fraternal” twins because they are the product of two zygotes (two different sperms and two different egg pairings) that occur at the same time.

For the purposes of NIPT, dizygotic twins are considered two separate pregnancies. These two babies have entirely different DNA sequences, whereas monozygotic twins have identical genomes. Therefore, when testing fetal DNA found in the mother’s blood, monozygotic twins are each expected to have the same risk of being born with a genetic condition. Dizygotic twins, however must be evaluated separately, which can be complicated if not enough of one twin’s DNA is present in the mother’s blood for accurate analysis. The estimated risk results of a dizygotic pregnancy must also be evaluated differently because probabilities differ when dealing with two entirely different fetuses, versus two fetuses with the same genome.

Knowing the Gender

One of the most exciting parts of non-invasive prenatal testing can be learning the gender of your babies. Of course, your doctor can arrange to keep this part of the results a secret from you if you’d like to wait until delivery, but some couples really enjoy preparing and imagining their new family. NIPT with multiples can often be more complex than with a single baby because it can lead to inaccurate results if a Y chromosome (meaning at least one of the twins is a boy) is detected from the mother’s blood. However, these tests and the ability to distinguish results have improved significantly in recent years, and it’s important to note that these results can also be corroborated with an ultrasound at a later time.

Ask Questions

It’s important to find a good prenatal care team that you trust, especially when you are facing a pregnancy with multiples. You’re going to have a lot of questions– that’s expected and totally normal. However, if you have specific questions about non-invasive prenatal testing or any potential genetic abnormalities in your babies, it may be a good idea to find a genetic counselor in your area that can recommend any additional testing, explain your results, and help you to plan a healthy, happy life for you and your growing family.


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