I have decided…
A. I do not want to have prenatal screening, now what?
Your health care provider will follow you as in any other pregnancy and make sure that both you and your baby are in the best possible health. Where available, you may be offered an ultrasound in the late first trimester (around 11-14 weeks gestation) for more accurate dating of your pregnancy, determination of twins, and early detection of major congenital anomalies. You may be offered an ultrasound in the second trimester at around 18-20 weeks gestation to look at the baby’s growth and anatomy.
B. I do not want to have prenatal screening but rather diagnostic testing, now what?
Your access to diagnostic testing will vary depending on where you live. If you, or in the case of a donor, the egg, will be 40 years or older at the time of birth you will likely have the option to choose diagnostic testing without prenatal screening. Talk to your health care provider about next steps. See page 15 for more on diagnostic testing, and for more on available genetic tests see Additional Resources.
C. I do want to have prenatal screening, now what?
Once you have decided to proceed with prenatal screening, the next step is for you and your health care provider to choose which testing option is right for you. The following two tables compare available screening tests. Some factors that affect which prenatal screening tests you can choose from are:
- Where you live
- Screening choices vary according to where you live. Your health care provider will discuss what is available.
- How far along you are in the pregnancy
- Accurate dating of your pregnancy, determined by a first trimester ultrasound, is important for accurate screening results. Dating based on last menstrual period (LMP) is not ideal, but if a first trimester ultrasound is not available this dating can be used.
- If the first prenatal visit with your health care provider is before 14 weeks (31/2 months) of pregnancy, options that may be presented to you include:
- First Trimester Screening (FTS)
- Enhanced First Trimester Screening (eFTS)
- Integrated Prenatal Screening (IPS)
- Serum Integrated Prenatal Screening (SIPS)
- Non-Invasive Prenatal Testing (NIPT)
- If the first prenatal visit with your health care provider is after 14 weeks (3-1/2 months) and before 21 weeks (~5 months) of pregnancy, options that may be presented to you include:
- Maternal Serum Screening (MSS)
- Non-Invasive Prenatal Testing (NIPT)
- The age of the pregnant person (or the egg in the case of a donated egg) when the baby is born
- If the pregnant person/egg will be 40 years of age or older at the time of birth, prenatal testing options include all screening tests above, in addition to the option of diagnostic testing
- If the pregnant person/egg will be 39 years of age or younger at the time of birth, prenatal options include all of the screening tests above, however NIPT would have to be paid for out of pocket and is not covered by Ontario Health Insurance Plan (OHIP). Diagnostic testing would not typically be offered at this time.
In addition to blood work, many screening tests also include an ultrasound measurement called the nuchal translucency (NT).
What is the NT measurement?
- The nuchal translucency (NT) is a fluid filled space at the back of every baby’s neck
- A larger NT measurement is associated with an increased chance for chromosome disorders like Down syndrome and some other genetic and non-genetic issues like congenital heart defect
- An ultrasound to measure NT is performed between 11 and 14 weeks of pregnancy and should be done at a certified ultrasound site. Your health care provider will tell you where an NT ultrasound can be obtained.
- An NT measurement of 3.5 mm or bigger is considered significant and would be a screen positive. A referral for genetic counselling will likely be offered.
Table 1 has an overview of the prenatal screening tests available in Ontario and how they compare to each other. Not all test options are available in all areas.
First Trimester Screening (FTS) and Integrated Prenatal Screening (IPS) are the most commonly offered prenatal screening tests. Both screen for Down syndrome and trisomy 18. IPS also screens for open neural tube defects. Results from FTS are available earlier in pregnancy.
Prenatal screening is moving toward results available earlier in pregnancy, allowing expectant couples more time for decision making and access to additional services. Already many centres are offering enhanced FTS (eFTS). This improved test performs better than traditional FTS with a higher detection rate and lower false positive rate. eFTS is expected to be just as good as IPS, but with results available earlier in pregnancy.
When access to a certified ultrasound site to measure the nuchal translucency is not possible, Serum Integrated Prenatal Screening (SIPS) may be offered.
A pregnant person who presents to their first prenatal visit at a later gestational age (after 14 weeks) may be offered Maternal Serum Screening (MSS).
An ultrasound is recommended for all pregnant persons at about 18-20 weeks of pregnancy to look at the baby’s growth and anatomy, regardless of prenatal screening results. While most babies are born healthy, 1 in 25 babies in Canada will be born with a difference that may require medical intervention. It is important to remember that no test can detect every type of congenital anomaly.
If you are expecting twins, prenatal screening is available but in general is not as good as for pregnancies with one baby. Also, not all screening options will be available for a twin pregnancy. Talk to your health care provider for more about screening in a twin pregnancy.
Table 1. Prenatal screening tests available and how they compare.
Updated April 2017
|First Trimester Screening (FTS)||Integrated Prenatal Screening (IPS)||Non-Invasive Prenatal Testing (NIPT)/cfDNA screening||Serum Integrated Prenatal Screening (SIPS)||Maternal Serum Screening (MSS)|
|Components of testAll screens use pregnant person’s age in chance assessments|
u/s = ultrasound
NT = nuchal translucency
|Gestational age at first trimester blood test||11 – 13 weeks + 6 days||11 – 13 weeks + 6 days||9-10 weeks onward||11 – 13 weeks+6 days||Not applicable|
|Gestational age at the NT ultrasound||11 – 13 weeks + 6 days||11 – 13 weeks + 6 days||Not applicable||Not applicable||Not applicable|
|Gestational age at second trimester blood test||Not applicable||15 – 20 week + 6 days||Not applicable||15 – 20 week + 6 days||15 – 20 week + 6 days|
|Detection rate||Meaning, how many pregnancies where the baby really does have Down syndrome will be flagged as increased chance (screen positive) by this test?|
|False positive rate||Meaning, how many pregnancies will this test flag as increased chance (screen positive) BUT the baby does NOT really have Down syndrome?|
|about 3-9%||about 2-4%||Less than 0.1%||about 2-7%||about 5%|
|Conditions screened for|| Down syndrome|
· Sex chromosome differences