Genetic Screening

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What is Prenatal Genetic Screening?

Prenatal genetic screening is offered to all clients during each pregnancy. Prenatal screening is a method of determining the chance a baby may or may not have Trisomy 21(T21, Down Syndrome) or Trisomy 18 (T18, Edwards’ Syndrome). Although rare, it is possible for any pregnancy to be affected by either of these syndromes regardless of family history. The chance of a baby having T21 or T18 increases with the age of the mother (or egg donor). 

Prenatal screening involves only ultrasound and maternal blood work, and is therefore considered of no risk to the pregnancy. Prenatal screening is not diagnostic, it does determine the chance/risk level of a pregnancy having T18 or T21. The only way to receive a ‘yes’ or ‘no’ about T21 or T18 during pregnancy is to have diagnostic testing, ie. Chorionic villus sampling (CVS) or amniocentesis. CVS and amniocentesis are generally reserved for pregnancies determined to be ‘high risk’ through the methods of prenatal screening. 

Prenatal screening is completely optional. Regardless of if you choose to proceed with prenatal screening or not, the routine pregnancy care you receive from your midwife will not be affected. If a chromosome difference is diagnosed, your midwife may recommend some changes to how the pregnancy is looked after or to the birth plan. 

Deciding if prenatal screening is right for your family is a deeply personal and difficult decision. It can be helpful when making this decision to answer some of the following questions: 

  • Would it be helpful for me to know if my baby has a chromosome difference during the pregnancy? 
  • Would I prefer to wait until my baby is born to know?
  • How would knowing or not knowing affect me emotionally throughout the pregnancy?
  • Would knowing if my baby has a chromosome difference help me to prepare for having a child that may require special care?
  • Would I consider interrupting the pregnancy if the result is confirmed by further diagnostic testing?

There are no right or wrong answers to these questions, only your answers, leading you to your personal decision regarding prenatal screening. 

Types of Prenatal Genetic Screening


Multiple Marker Screening

Both of these screenings are OHIP-funded and will provide information about the chance of having a baby with Trisomy 21 or Trisomy 18. Both screenings can be ordered by your midwife, who can provide you with the required requisitions upon request. 

  • enhanced First Trimester Screening (eFTS): done between 11+2 and 13+3 weeks of pregnancy, more accurate than STS
  • Second Trimester Screening (STS): done between 14+0 and 20+6 weeks, less accurate than eFTS


Non- Invasive Prenatal Testing (NIPT)

NIPT can be done anytime after 9 or 10 weeks of pregnancy (depending on the lab). NIPT results are determined only from a maternal blood sample. NIPT will provide information about the chance of having a baby with Trisomy 18, Trisomy 21, Trisomy 13 (Patau syndrome), and sex chromosome differences. NIPT is considered the most accurate method of prenatal screening when compared to eFTS and STS. NIPT is only OHIP funded in certain situations. One situation is If you receive a ‘positive screening’ from eFTS or STS, NIPT will then be OHIP funded. For all other clients, NIPT will be paid out of pocket with prices ranging from $500 to over $700, depending on the testing lab. Additionally, midwives are not able to order NIPT at this time. As such, if you decide NIPT is the right prenatal screening method for your family, you will have to request the requisition from your family physician. 


Diagnostic Testing

If you receive a ‘high risk’ result, your midwife will complete a referral to the regional specialists who will provide you with further diagnostic testing options and genetic counseling. A ‘high risk’ result from any form of prenatal screening, will make you eligible for diagnostic testing.The two methods of diagnostic testing currently available include: 

  • Chorionic Villus Sampling (CVS): CVS is available from 11 to 14 weeks of pregnancy. CVS involves removing a small piece of the placental tissue, which is then sent to the lab for further testing. 
  • Amniocentesis: Amniocentesis can be done anytime after 15 weeks of pregnancy. Amniocentesis involves removing a small amount of amniotic fluid that surrounds the baby, which is then sent to the lab for further testing. 

Deciding to proceed with prenatal screening or not is a difficult personal decision. Below are additional resources to assist you in making this decision. Please feel free to ask your midwife any remaining questions you may have.