Genetic Counseling in a Twin Pregnancy

Contributors: Michelle Pacione, EdM, MS, CGC, is a genetic counselor with the Center for Fetal Medicine and Reproductive Genetics at Brigham and Women's Hospital. Carolina Bibbo, MD, is a maternal-fetal medicine specialist and director of the Comprehensive Care Center for Multiples at the Brigham.

For many families expecting twins, a key member of their care team is the genetic counselor. In this article, we explain the role of this medical professional and how you may benefit from their guidance.

One of the first questions that patients have early in their pregnancy is if their baby or babies are at risk for a genetic disorder. A genetic disorder is a disease caused by changes in the DNA, which is a molecule that includes instructions for development, growth and reproduction.

Inside each cell of the body, DNA is contained within thread-like structures known as chromosomes. Each person has 23 pairs of chromosomes. For each pair, one chromosome comes from the mother and the other from the father.

There are three main types of genetic disorders:

  • Chromosome abnormalities usually occur without any family history. An example is Down syndrome, which occurs when cells contain an extra copy of chromosome 21.
  • Single-gene disorders usually have a higher risk of being passed from parents to children. They may be inherited from one parent or from both parents, or they occur spontaneously in the pregnancy. Examples include cystic fibrosis and sickle cell anemia.
  • Copy number variants are deletions of duplications of genetic material that can lead to genetic disease.

Understanding genetic disorders, how they may affect your babies and whether genetic testing may be helpful is not easy. That's where a genetic counselor comes in.

Your Guide to Genetic Disorders and Testing

A genetic counselor is a medical professional with expertise in genetic disorders and testing. Michelle Pacione says her responsibilities span three primary areas.

"One is educating parents and families about genetic disorders and when testing may be needed," she says. "Two is psychosocial counseling — helping them to make sense of any complications that are found and set up a plan that works for them. And three is coordinating follow-up care, such as scheduling additional tests or making referrals to appropriate specialists."

At the Comprehensive Care Center for Multiples, every patient has the option to meet with a genetic counselor who specializes in pregnancies with twins and higher-order multiples. According to Pacione, this is critical because different forms of genetic testing use different technologies. "If you're not experienced with twin pregnancies or how the technologies relate to twins," she says, "then you may order a test that actually isn't the most appropriate for the condition you're looking for."

A genetic counselor will discuss your various testing options with you. They will also help you decide which test or tests are right for you and make sense of the results. Genetic testing is never required — whether to proceed is entirely up to you.

Screening Tests vs. Diagnostic Tests

One type of genetic testing is screening tests, which can reveal whether your babies are at higher or lower risk for genetic disorders. They do not, however, provide a final diagnosis. These tests include:

  • Noninvasive prenatal screening uses a blood sample from the mother to look for chromosome abnormalities.
  • Carrier screening uses a blood or saliva sample from each parent to check if they carry certain genetic disorders that can be passed on to their children.
  • Ultrasound uses sound waves to see how your babies are growing. The ultrasound typically done at 10 to 13 weeks is an assessment of nuchal translucency, which checks the area at the back of each baby's neck for extra fluid. Another ultrasound done at 18-20 weeks, the fetal anatomic survey, scans each baby's body to look for structural problems, some of which may be related to genetic disorders.

"The combination of a blood test and the nuchal translucency ultrasound is really important with twins," Dr. Carolina Bibbo says. "If the screening blood test comes back as high risk, it doesn't point to which baby might be affected. The ultrasound measurement of the nuchal translucency thickness can certainly improve the detection of a problem and better identify the affected twin."

If screening detects a possible problem, your genetic counselor may suggest options for diagnostic tests. "In this case," Pacione says, "we're obtaining some DNA from the pregnancy to better understand if the problem is related to genetics."

The two most common diagnostic tests are:

  • Chorionic villus sampling (CVS) uses a sample of tissue from the placenta to look for problems.
  • Amniocentesis (amnio) uses a sample of amniotic fluid, the liquid that surrounds the babies inside the womb, to check for problems.

Both CVS and amnio carry a less than 1 percent risk of miscarriage, according to Pacione. Still, you may find greater peace of mind having these tests done at an experienced program like the Center for Multiples, which conducts them many times a day.

Making the Complex a Little Simpler

What happens if diagnostic testing shows one or both of your babies has a genetic disorder? Your genetic counselor will help you decide on next steps, including possible referrals to specialists.

Depending on the condition, your genetic counselor may recommend further imaging tests, including:

  • Ultrasound
  • Fetal echocardiogram: an ultrasound that checks each baby's heart for problems
  • Magnetic resonance imaging (MRI): a test that makes a series of cross-sectional images of each baby's body

All of the above tests generally are not done until the second trimester.

"Dealing with a genetic disorder can be complicated," Pacione says. "Helping parents and families figure out what pieces of information are most important, making it digestible and setting up a plan that aligns with their goals and values is what my job is all about."

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