Understanding First Trimester Screening
Welcoming a new life into the world is a period of excitement and anticipation. However, for many expecting individuals and their families, this journey may also be filled with questions, concerns, and the need for informed decision-making. This is where the important role of prenatal screening comes into play, particularly in the first trimester of pregnancy.
In Canada, prenatal screening is offered to all pregnant people as a non-invasive way to assess the risk of certain chromosomal differences, including trisomy 13, trisomy 18, and trisomy 21.
Through a combination of ultrasound and bloodwork, healthcare providers can help families understand the likelihood of these genetic conditions, shaping how they prepare emotionally for the pregnancy and postpartum period.
While screening provides valuable insights, it isn’t diagnostic and it cannot test for all genetic or non-genetic health issues.
In this blog post, we’ll explore the significance of prenatal screening, the process involved, and insights into how it empowers families to make informed decisions during this pivotal time, ensuring that they feel supported every step of the way.
Dating Ultrasound
WHEN: Between 7-9 weeks of pregnancy
This transabdominal and transvaginal ultrasound uses sound waves to:
Confirm gestational age
Estimate the due date
Measure crown-rump length
Assess fetal cardiac activity
Detect multiples
Prenatal Genetic Screening
Prenatal genetic screening for chromosomal differences is a significant consideration for expectant parents, as it can provide crucial information about a baby's health before birth.
For many families, knowing whether their baby has an increased chance of a chromosomal difference can be important for emotional preparation and decision-making.
A positive screen result may prompt parents to consider additional testing and help them prepare for the possibility of a baby with special needs. On the other hand, some parents may choose to decline screening to avoid the anxiety of potential false positives or difficult decisions regarding continuing the pregnancy.
Ultimately, the choice about whether to pursue screening is incredibly personal and can significantly impact how parents emotionally navigate their pregnancy journey.
Chromosomal Differences
Many of these tests screen for a combination of these differences:
Trisomy 13 (Patau Syndrome): 3 copies of chromosome 13 instead of 2Trisomy 18 (Edward Syndrome): 3 copies of chromosome 18, instead of 2Trisomy 21 (Down Syndrome): 3 copies of chromosome 21, instead of 2
Enhanced First Trimester Screen (eFTS)
WHEN: 11 weeks and 2 days to 13 weeks and 3 days of pregnancy
HOW: Nuchal translucency ultrasound and blood test screening for multiple markers
SCREENS FOR: Trisomy 18 and Trisomy 21
eFTS is exclusively available for singleton pregnancies (pregnancy with only one baby). Some situations where an eFTS cannot be done are:Twin pregnancy - NIPT done instead
Vanishing twin or co-twin demise
A negative result means that your baby has a lower chance of having trisomy 18 or 21. It doesn’t mean that there isn’t a chance of these conditions though - it's just unlikely.
A positive result means there’s a higher chance of your baby having trisomy 18 or 21. Most people with this result don’t actually have a baby with one of these conditions. Further testing may need to be done for more certainty.
Nuchal Translucency Ultrasound
WHEN: 11-14 weeks
HOW: Ultrasound
This ultrasound measures a pocket of clear fluid at the back of the baby's neck, called the nuchal translucency. It's normal for every baby to have some fluid at the back of the neck at this time. When a baby has more fluid, there's a higher chance for the baby to have a genetic condition or another health issue.
This ultrasound can give you an accurate due date, and can tell you how many babies you are expecting. The ultrasound also looks at the baby's body parts, like the heart and the brain.
Non-Invasive Prenatal Testing (NIPT)
WHEN: After 9-10 weeks of pregnancy
HOW: Blood test
SCREENS FOR: Trisomy 13, Trisomy 18, Trisomy 21
During a pregnancy, the baby's placenta releases cell-free DNA (cfDNA) into the blood stream. The NIPT assesses this cfDNA and depending on the amount of cfDNA fragments, a chromosomal difference may be suspected. If a NIPT test is done, there is no need for the eFTS (blood work portion) but a 11-14 week ultrasound is still recommended.
A NIPT is not recommended when:
Pregnancy started a twin pregnancy, but one of the twins was lost (vanishing twin syndrome/co-twin demise)
A pregnancy with more than two babies
In Ontario, OHIP covers the cost of the NIPT test based on the following criteria:
The pregnant person will be 40 or older at the time of birth
Past pregnancy with trisomy 13, 18, or 21
Current twin pregnancy
If you don’t meet the above criteria, but are still interested in this test - your obstetrical care provider or family doctor will arrange it for you.
NIPT results are reported as either low risk or high risk. Similar to the eFTS, the results cannot definitely say if the baby has a chromosomal difference - but it is more accurate than the eFTS.
A “high risk” result in a high risk population (ex: maternal age over 35, positive eFTS, ultrasound abnormalities) is more likely to be a true result, than in a low risk population. Genetic counselling may be recommended after this result, as well as the possibility of further testing like chorionic villus sampling or amniocentesis to know for sure.
Sometimes the NIPT may fail, and no result is given. This may be due to numerous factors such as:
Early gestational age
High Body Mass Index (BMI)
In vitro fertilization (IVF)
Multiple gestation pregnancy
Other options for testing include another NIPT, doing an eFTS or second trimester screen (STS), 18-22 week anatomy scan, and/or referral to genetic counselling where they may discuss diagnostic testing.
Chorionic Villus Sampling (CVS)
WHEN: 11 - 14 weeks
HOW: Ultrasound and invasive tissue sampling
RISK: Miscarriage
Unlike the others, CVS is a diagnostic test where a piece of tissue is taken from the placenta and sent to a lab for further testing. The placental tissue usually contains the baby’s genetic information.
A CVS is considered in the following situations:
Positive eFTS screen
“High risk” result from NIPT
NIPT failed after 2 or more times
Nuchal translucency ultrasound detected more fluid
Hereditary genetic condition
Next Steps
First trimester screening plays a vital role in prenatal care, offering expectant parents valuable insights into their baby's health and development. By understanding the tests available and the information they provide, families can make informed decisions and prepare for the journey ahead. It’s essential to maintain open communication with your healthcare provider, ask questions, and discuss any concerns you may have.
Remember, this screening is just one part of a comprehensive approach to a healthy pregnancy. Embrace the knowledge you gain from this process, and rest assured that every step taken is a step toward ensuring the best possible outcomes for both you and your baby.