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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". If your test result is "screen negative", you might have questions about what this means and what the next steps are.
Screen Negative
A "screen negative" result means there that there is a lower chance for the pregnancy to have trisomy 21 (Down syndrome) or trisomy 18 (Edwards syndrome). A lower chance does not mean "no chance".
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A number on the report gives your specific chance that the baby has trisomy 21. For example: You got a "screen negative" result for trisomy 21, and the number on your report is 1 in 500. This means out of 500 people who get this result, 1 will have a pregnancy with trisomy 21, and the other 499 people will not (see figure). You can also think of it as a 0.2% chance for the baby to have trisomy 21, and a 99.8% chance for the baby to not have trisomy 21.
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The screening test has a cut-off of 1 in 350 for trisomy 21. If you got a "screen negative" result, it is because the chance for the baby to have trisomy 21 was calculated to be less than 1 in 350.
You will get routine pregnancy care if there are no other concerns with your pregnancy.
You can have a 18-22 week (detailed anatomy) ultrasound to check how the baby is growing and developing. This ultrasound is offered to all pregnant people, no matter what their prenatal genetic screening results are.
Many people do not do any more testing after they get a "screen negative" result. However, some people will think about having another screening test called Non-invasive Prenatal Testing (NIPT). Publicly funded NIPT is not available for vanishing twin pregnancies.
Contact Us
Prenatal Screening Ontario
CHEO Research Institute
Centre for Practice-Changing Research Building
401 Smyth Road
Ottawa, ON K1H 8L1
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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Virtual educational sessions on prenatal screening and nuchal translucency quality assurance are available upon request for health‑care providers.
