Non-Invasive Prenatal Screening for Twin Pregnancies

By on August 3, 2021

A number of factors make twin pregnancies higher-risk, higher-cost, and harder-to-manage than singleton pregnancies. These factors include generally higher gestational age and fewer suitable options for screening and diagnostic tools, which are becoming increasingly important for obstetricians to find solutions to due to the rising rate of twinning in the United States.

Fortunately, as Dr. Rebarber describes in a recent article in the journal Prenatal Diagnosis, non-invasive prenatal screening (NIPS) shows promise in determining fetal number and health risks early in pregnancy, and monitoring development throughout gestation.

What is Non-Invasive Prenatal Screening?

NIPS is a technique that involves testing the mother’s blood to find free-floating DNA, or DNA that’s not encased in a cell. So-called cell-free DNA (cf-DNA) comes from both the mother and the fetus(es), allowing doctors to analyze fetal DNA to determine a number of things:

  • The number of fetuses
  • The number of placentas
  • Whether the fetus(es) are at risk for aneuploidy
  • If there is a vanishing twin

Why Use Non-Invasive Prenatal Screening?

NIPS generally provides accurate results and may be safer than invasive diagnostic techniques such as chorionic villus sampling (CVS) or amniocentesis, which have been shown to slightly increase the risk of pregnancy loss. In addition, this test is available beginning late in the first trimester (as early as 9-10 weeks gestation), allowing doctors to monitor the development of twins early on, providing advanced notice of the condition of twin pregnancy and likely improving clinical management and care.

This testing compounds the benefits of other non-invasive techniques, such as ultrasound, and can be used as an adjunct when ultrasound results are inconclusive or unavailable.

Importance of Determining Zygosity

Zygosity refers to how many sets of chromosomes are in the womb. Twins are born from either one or two eggs, creating either a shared set of genes or two distinct genomes, respectively. Monozygotic pregnancies are identical twins and dizygotic pregnancies are fraternal twins.

It’s important for your maternal fetal medicine specialist to identify whether it’s an identical pregnancy or not for many reasons. In identical pregnancies, if an anatomic abnormality or other condition is detected, it is more likely that both fetuses are affected, which can affect the mother’s decisions about pregnancy care and clinical management. This is especially true when it comes to aneuploidy, which is when there are either extra or fewer chromosomes than expected. This is the cause for several conditions, perhaps most well-known is Down syndrome, which occurs when there are three pairs of chromosome 21.

Importance of Determining Chorionicity

The number of placentas in a twin pregnancy is another important piece of information, as twins who share a single placenta are at a higher risk for several rare conditions that will require additional testing and close monitoring.

Usually, an ultrasound can identify whether a twin pregnancy is either monochorionic (1 placenta) or dichorionic (2 separate placentas), although not every patient is able to get this critical test performed, because they did not begin prenatal care early enough. According to Dr. Rebarber, “Over 20% of women entered prenatal care in the second trimester or later and this proportion was even greater in marginalized and vulnerable populations.”

Using cf-DNA is not a perfect replacement for ultrasound testing, although it can help rule out monochorionicity if the twins are not identical. In identical twin pregnancies, approximately 75% are monochorionic and 25% are not, however, in fraternal (or nonidentical twins) all cases are dichorionic.

Monitoring Vanishing Twins

Especially prevalent in pregnancies, sometimes as a result of in vitro fertilization, a vanishing twin is a demised twin that can affect certain test results, cause abnormal bleeding, or result in spontaneous preterm birth. NIPS can identify a vanishing twin to explain these or other conditions and offer a clinical path forward for treatment.

NIPS Helps People Plan Pregnancy Care

Although not a replacement for other established monitoring and diagnostic testing techniques particularly in the setting of known fetal abnormalities, NIPS may be  a helpful complement to ultrasound in the routine setting This test facilitates suspicion of adverse conditions, giving mothers information about the condition of their pregnancy and give them time to process and make decisions about invasive diagnostic testing..

The key to a healthy twin pregnancy is early and consistent treatment. Don’t wait to schedule your first appointment with a maternal fetal medicine specialist in New York City to ensure both you and your baby grow safely and healthily together. Call us or contact us online to start your prenatal care plan today.

Maternal Fetal Medicine blogs are intended for educational purposes only and do not replace certified professional care. Medical conditions vary and change frequently. Please ask your doctor any questions you may have regarding your condition to receive a proper diagnosis or risk analysis. Thank you!

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