Our early scan showed a vanishing twin. Can we still have Non-Invasive Prenatal

[Anonymous]

Our early scan showed a vanishing twin. Can we still have Non-Invasive Prenatal Testing?

[Genetic Counselor]

You can, but the result needs careful interpretation, and your care team must know about the vanishing twin before the test is done.

What a vanishing twin is

Sometimes an early scan shows two pregnancy sacs, but one baby stops growing within the first few weeks. Doctors call this a vanishing twin. It is more common than many people realize.

It is seen more often in pregnancies conceived through in vitro fertilization, because very early scans are routine in that setting. In most cases, the surviving baby continues to grow normally.

Why it affects Non-Invasive Prenatal Testing

Non-Invasive Prenatal Testing reads small fragments of placental DNA in the mother’s blood. Even after the lost twin stops developing, its placental tissue can keep releasing DNA into the mother’s bloodstream for some time.

Pregnancy losses this early are often caused by a chromosome condition in that baby. If so, the test may pick up the lost twin’s DNA and report a high risk, even though the surviving baby is healthy.

How long this effect lasts

Published reports show the leftover DNA can persist for several weeks. In one documented case, it was still present nine weeks after the loss. Researchers have not agreed on a waiting period after which the test becomes fully reliable. This is why no one can promise that waiting a fixed number of weeks will remove the problem.

What a sensible plan looks like

Team Genetidoc covers a vanishing twin history in counseling before the test. A typical approach includes:

A careful first-trimester scan. The nuchal translucency measurement and early anatomy check of the surviving baby give information about that baby alone, unaffected by the lost twin’s DNA.
An honest discussion of what a result could mean. If you go ahead with Non-Invasive Prenatal Testing, you should know in advance that a high-risk result might come from the lost twin.
Confirmation before any decision. Any high-risk result should be checked with a diagnostic test of the surviving baby, such as amniocentesis, before any decision is made about the pregnancy.

An illustrative example (composite, details changed)

A 34-year-old woman conceived through in vitro fertilization. Her 7-week scan showed two sacs, but by 9 weeks only one baby was growing. At 11 weeks, she asked about Non-Invasive Prenatal Testing.

During counseling, she learned that DNA from the lost twin could still be in her blood. Together with the genetic team, she agreed on a plan: a careful first-trimester scan, and diagnostic testing to confirm any high-risk result before acting on it. Knowing this in advance meant that a confusing report would not catch her off guard.

What you should do

Tell your doctor and the laboratory about any second sac that stopped growing, even if it seemed minor at the time.
Keep your early scan reports, because the timing of the loss helps with interpretation.
Do not make any decision based on a screening result alone.

[Author]

Posts