Vanishing Twins Screening Positive Result

If you have a "vanishing twin" pregnancy, your prenatal screening test might have been a 11-14 week (nuchal translucency) ultrasound followed by Second Trimester Screening (STS). Or you might have had Second Trimester Screening (STS) on its own. Either way, your results are either "screen negative" or "screen positive".

Choose your result to learn more about what it means and next steps:

Screen Positive

A "screen positive" result means there that there is a higher chance for the baby to have trisomy 21 (Down syndrome) or trisomy 18 (Edward syndrome). It does not mean that the baby definitely has one of these chromosome differences.

Understanding your chance

A number on the report gives your specific chance that the baby has trisomy 21 or trisomy 18.

For example:

You got a "screen positive" result for trisomy 21, and the number on your report is 1 in 200.

This means out of 200 people who get this result, 1 will have a pregnancy with trisomy 21, and the other 199 people will not (see figure). You can also think of it as a 0.5% chance for the baby to have trisomy 21, and a 99.5% chance for the baby to not have trisomy 21.

Understanding the cut-off

Trisomy 21:

The test has a cut-off of 1 in 350 for trisomy 21. If you got a "screen positive" result for trisomy 21, it is because the chance for your baby to have trisomy 21 was calculated to be higher than 1 in 350.

Trisomy 18:

The test has a cut-off of 1 in 200 for trisomy 18. If you got a "screen positive" result for trisomy 18, it is because the chance for your baby to have trisomy 18 was calculated to be higher than 1 in 200.

Next steps

If you have a "screen positive" result, your health-care practitioner should offer you options for more testing.

Non-Invasive Prenatal Testing (NIPT)
- Non-Invasive Prenatal Testing (NIPT) will now be funded by public funds in your situation. NIPT is a more accurate screening test compared to the test you have already done. This test can be arranged by your health-care provider. NIPT is not possible if you had a "vanishing" twin pregnancy.
Diagnostic Testing
- Publicly-funded diagnostic testing (usually through chorionic villus sampling or amniocentesis) is the only way to know for sure if your baby has a chromosome difference during the pregnancy. If you wish to have a diagnostic test, you need to be referred to a genetics specialist in your area.
No more testing during the pregnancy
- Some people do not wish to have any more testing during the pregnancy. If there are other concerns, there might be testing offered after the baby is born.

You can also have a 18-22 week (detailed anatomy) ultrasound to check how the baby is growing and developing. This ultrasound can give you extra information. A normal ultrasound does not guarantee a healthy baby.

If your result is "screen positive" and the nuchal translucency measurement on your ultrasound is "increased" (3.5 mm or more), your health-care provider should speak to you about the option of being referred directly to a genetics specialist.

Most of the time when there is a "screen positive" result, the baby does not actually have trisomy 21 or trisomy 18. This means that the result is usually a false positive.

Contact Us

Prenatal Screening Ontario
CHEO Research Institute
Centre for Practice-Changing Research Building
401 Smyth Road
Ottawa, ON K1H 8L1

pso@bornontario.ca

Information line
Phone: 613-737-2281
Toll-Free: 1-833-351-6490

Monday to Friday, 9:00 AM to 3:00 PM EST

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