- Hadlock, F.P., Deter, R.L., Harrist. R.B., Park, S.K. (1982). Fetal biparietal diameter: a critical re-evaluation of the relation to menstrual age by means of real-time ultrasound. J Ultrasound Med 1(3):97–104. DOI: 10.7863/jum.1982.1.3.97
- Daya, S. (1993) Accuracy of gestational age estimations by means of fetal crown-rump length measurement. Am J Obstet Gynecol 168(3 Pt 1):903–8. DOI: https://doi.org/10.1016/S0002-9378(12)90842-X
Screening for Vanishing Twins Guide
Nuchal Translucency and Second Trimester Screening (NT + STS) is a screening test for trisomy 21 and trisomy 18 consisting of a nuchal translucency ultrasound in the first trimester. The blood work for this test is done in the second trimester of pregnancy, not on the day of the ultrasound. This test is offered mainly to individuals who have a singleton pregnancy following a vanishing twin/co-twin demise.
Step-by-step instructions are outlined below to provide guidance to health-care providers for how to order this test in Ontario.
It is important to know that enhanced First Trimester Screening (eFTS) and publicly funded Non-invasive Prenatal Testing (NIPT) are not available for vanishing twin pregnancies.
Step 1
Complete the Multiple Marker Screening Requisition
Multiple Marker Screening (MMS) is done by three provincial laboratories: Trillium Health Partners - Credit Valley Hospital, Mount Sinai Hospital and North York General Hospital. Check out our interactive map to learn which of the three requisitions you should use.
The Multiple Marker Screening Requisition must be filled out prior to the NT ultrasound. Please note that missing information decreases the quality of the screen and can potentially delay the results.
Demographics Section
The pregnant individual’s name, date of birth, health card number, address, and phone number should be completed.
"Test Requested" Section
Check off the "NT + Second Trimester Screening" option and fill in the date of the STS blood draw.
- NT+ STS is available to individuals who had a "vanishing twin"/co-twin demise identified prior to the NT ultrasound. This test may also be available to individuals who had a NT ultrasound, but missed the time window for the eFTS bloodwork.
- The nuchal translucency (NT) ultrasound is only available when the crown-rump length measurement (the measurement of the length of embryo from top of the head to the bottom of the torso) at the time of the ultrasound is between 45 and 84 mm. This corresponds to a gestation between 11 weeks 2 days' to 13 weeks 3 days' gestation, based on published charts of Hadlock and Daya1.
- An STS blood sample is needed between 14 weeks 0 days' and 20 weeks 6 days' gestation. However, the blood sample cannot be drawn until 8 weeks after the demise. Based on the estimated date of demise, calculate the date after which the pregnant individual can have their blood drawn. Write this date in the space provided in the “STS + NT” section.
If the demise was estimated to have occurred at 10 weeks 0 days', the pregnant individual could have the STS blood draw after 18 weeks 0 days' (and up to a maximum of 20 weeks and 6 days').
"Clinical Information" Section
Accurate and complete information in this section will increase the quality of the screening results by reducing the chance of false positives and false negatives.
- I have some questions about you to ask to make sure your screening is as accurate as possible.
Race is a way of grouping people based on biological traits like skin colour, hair texture, language, or where they are from. It is a social construct, meaning these groups were created by humans. Examples of race categories are: Asian, Black, Middle Eastern, White, among others. While race is sometimes used as a proxy (or a stand-in) for ethnicity or genetic ancestry, these terms differ in definition. Ethnicity generally refers to social and cultural categories, whereas genetic ancestry refers to genetic origins.
Why is race collected?
Race is collected because studies have shown that the baseline (or starting) concentration of the serum markers used in MMS differ between races. Therefore, the screening algorithm makes adjustments based on the pregnant individual’s race, with the goal to make screening more accurate for everyone.
It is important to remember that race itself, as a social construct, is not responsible for the observed differences in serum marker concentrations. As we learn more about what truly causes the differences (e.g. social and structural factors, the impact of structural racism, and the biology behind the markers, including genetic ancestry), and, as we learn more about the experience of reporting one’s race, the approach may evolve. For now, race is being collected and used as a proxy for these unknown root causes.
How should race be collected?
Ideally, the pregnant individual self-reports their race on the requisition by checking all categories that apply or writing another race. Health-care providers should not choose a race for them. Refer to the table below for definitions of each race category.
Is sharing race mandatory?
Pregnant individuals can decline to report their race information and screening can still be done; however, the accuracy of the screening may be affected. Without race information, the calculated risk may over- or underestimate the true chance for the baby to have Down syndrome or trisomy 18.
Where is race data stored?
Race data is used in the screening software and included in the report for the ordering health-care provider. This information is also shared with BORN Ontario and is protected by law.
What if the race information needs to be corrected or if an individual wants it removed?
If the race on the screening report is not accurate, the health-care provider can contact the screening laboratory for further information. Sometimes, certain races are grouped into an overall category to be used in the screening algorithm, which means a person’s exact race might not be on the report but rather, a broader category. It is recognized that broad race categories are not ideal and do not reflect the diversity of individuals, yet this is a limitation of the screening software currently.
Other times, the wrong race may in fact have been reported on the requisition. In this case, the laboratory can usually correct this information and issue a new report.
If a pregnant individual would like their race removed entirely from the screening report, the laboratory would categorize the individual in the “other” grouping. This might affect the accuracy of the screening result.
Table 1. Current racial groups that are collected for MMS. *PSO, in collaboration with the MMS laboratories, is exploring the ability to expand the racial categories collected
|
Race Category |
Description/examples |
|
Black |
African, Afro-Caribbean, African-Canadian descent, etc. |
|
Asian |
Central Asian (Kazakhstan, Turkmenistan, Uzbekistan, Kyrgyzstan and Tajikistan and other Central Asian descent)
East Asian (Chinese, Korean, Japanese, and other East Asian descent),
South Asian (Afghanistan, East Indian, Pakistani, Bangladeshi, Sri Lankan, Indo-Caribbean, etc.)
Southeast Asian (Filipino, Vietnamese, Cambodian, Thai, Indonesian, other Southeast Asian descent) |
|
Indigenous: |
First Nations, Métis, Inuit descent |
|
White |
European descent
Middle Eastern (Arab, Persian, West Asian descent, e.g. Afghan, Egyptian, Iranian, Lebanese, Turkish, Kurdish, etc.) |
|
Other |
Another race category (write in response) |
Indicate the pregnant individual’s weight, including units (pounds (lbs) versus kilograms (kg)), at the time when this requisition is completed. This information is used for screening marker adjustments. A weight discrepancy of 10 or more pounds (4.6 kilograms) has a significant impact on the risk for trisomy 21 for pregnant individuals with a risk close to the screening cut-off.
- What do you think is your current weight?
- Do you think the weight your provided is accurate within 5 pounds?
The Last Menstrual Period (LMP) is not needed. For NT+STS, the crown-rump length measurement at the time of NT ultrasound will be used instead of the LMP, to establish the gestational age of the fetus and determine the appropriate time for bloodwork in the second trimester.
Insulin-Dependent Diabetes Mellitus History
Indicate if Insulin-dependent Diabetes Mellitus was diagnosed prior to the individual’s current pregnancy. Individuals with insulin-dependent Diabetes Mellitus have been shown to have different concentrations of screening markers compared to those who do not have this diagnosis. Screening marker adjustments must be made to account for these differences.
No screening marker adjustments are made for pregnant individuals with a history of gestational diabetes or preexisting diabetes that is not managed with insulin.
- Do you have a diagnosis of diabetes?
- Was the diabetes diagnosed before the current pregnancy?
- Do you use insulin to manage the diabetes?
Cigarette Smoking / Vaping History
Indicate if the pregnant individual has smoked or vaped any amount of nicotine at any point in the current pregnancy. Individuals with a history of smoking or vaping nicotine have been shown to have different concentrations of screening markers compared to those who do not have this history. Screening marker adjustments must be made to account for these differences.
If the requisition does not include information about the smoking or vaping history, the pregnant individual is considered a non-smoker. Due to lack of published data, pregnant individuals exposed to second-hand smoke are considered non-smokers.
- Have you smoked any cigarettes or used a nicotine vaping device at any point in the current pregnancy?
In Vitro Fertilization (IVF) History
If the pregnancy was conceived using in vitro fertilization (IVF), indicate the egg retrieval date and date of birth of the egg donor (either self or non-self). If the egg donor's full date of birth is unknown, indicate the year of birth. If only the year of the donor date of birth is provided, July 1 of the provided year is used as the date.
This information is requested because the age of the egg at the date of delivery influences the screening risk. In addition, screening marker levels vary between IVF and natural conceptions and screening marker adjustments must be made to account for these differences.
- Did the current pregnancy happen through in vitro fertilization (IVF)?
- What was the egg retrieval date?
- Were your own eggs used for this pregnancy, or was there an egg donor involved?
- Do you know the date of birth of the egg donor; at minimum, the year of birth?
"Ultrasound Information" Section
Be aware that only the sonographer/physician performing the NT scan completes the "Ultrasound Information" section when ordering NT + STS.
Step 2
Complete a Nuchal Translucency Ultrasound Requisition
Visit our interactive map to find facilities offering NT ultrasound in Ontario. You can contact the ultrasound facility of interest to obtain the centre-specific ultrasound requisition.
Step 3
Provide Requisitions to the Pregnant Individual
- Give both the completed nuchal translucency ultrasound requisition and Multiple Marker Screening requisition to the pregnant individual.
- If your office is booking the NT ultrasound, it is helpful to fax the MMS requisition with the ultrasound requisition to the ultrasound facility. It is important that the ultrasound facility has both of these requisitions at the time of the ultrasound appointment, either from your office or from the pregnant individual.
- Check if additional requisitions are required by the ultrasound and blood work facilities, such as the Ministry of Health and Long Term Care Laboratory Requisition.
Step 4
Provide Step-by-Step Instructions to the Pregnant Individual
- Provide clear instructions to the pregnant individual for how to navigate the different steps involved in obtaining NT + STS. Please note that Prenatal Screening Ontario does not organize prenatal genetic screening tests.
- If the pregnant individual misplaces it after the NT ultrasound, another copy of the completed Multiple Marker Screening requisition (including the ultrasound section) must be obtained from the ultrasound facility, whenever possible. Contact Prenatal Screening Ontario or the Multiple Marker Screening lab to discuss options if a completed Multiple Marker Screening Requisition cannot be obtained.
Step 5
Obtain Report and Check for Accuracy
- Results will be reported to you by the MMS lab generally within 5 business days after the receipt of the blood sample. It is important to take into account the time lag that can occur between the time when the report is ready and when it is received by your office.
- If you do not receive the prenatal screening report, we encourage you to follow up with the MMS lab directly. Prenatal Screening Ontario does not have access to screening results of individuals.
- Review the screening report for accuracy. If any errors or omissions are noted, contact the MMS Laboratory to determine if an amendment to the report is needed.
When is a Report Amended?
A report is considered “amended” if changes are made after the report has been issued to the health-care provider. Amending reports is necessary to correct errors that include data being inaccurately provided on the requisition. Some examples are inaccurate racial origin identification, inaccurate diabetes status, and correction of weight, due to either wrong units (i.e. pounds or kilograms) or difference in weight of more than 5 - 10 pounds (2.3 - 4.6 kilograms)1 from that given on the requisition.
1North York General Hospital and Mount Sinai Hospital labs will amend a report if there is a difference in weight of more than 10 pounds. The Trillium Health Partners lab will amend a report if there is a difference in weight of more than 5 pounds.
How is a Report Amended?
Health-care providers can write the amendment(s) directly on the screening report and fax it back to the Multiple Marker Screening Laboratory, indicating that an amendment is required as per the enclosed report. Alternatively, health-care providers can fax a note with the pregnant individual's name, date of birth, health card number, and details related to the amendment request, including relevant clinical information.
Step 6
Communicate Results and Next Steps
- Once the report is received, results should be communicated in both “screen positive” and “screen negative” scenarios, including the specific risk estimate provided on the report.
- Ensure continuity of care - forward the report to any other health-care providers involved in the care of the pregnant individual.
The results of the nuchal translucency ultrasound will be sent separately by the ultrasound facility. If the nuchal translucency measurement is increased (3.5 mm or above), inform the pregnant individual of this result as soon as possible and offer a referral for genetic counselling irrespective of the eFTS results.
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Prenatal Screening Ontario
CHEO Research Institute
Centre for Practice-Changing Research Building
401 Smyth Road
Ottawa, ON K1H 8L1
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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.