Prenatal Screening for Twin Pregnancies

It is recommended that you include the following information in your discussion with individuals who are pregnant with twins, and who wish to have screening for trisomy 21 (Down syndrome) and trisomy 18 (Edwards syndrome):

1. Offer Non-Invasive Prenatal Testing (NIPT) as an OHIP-funded option. 

2. Offer a 11-14 week ultrasound (including nuchal translucency (NT)) where available, as an important standard of early pregnancy care.

FTS is no longer offered in Ontario for twin pregnancies. Samples for FTS in twin pregnancies will not be processed by the provincial laboratories.

The International Society for Prenatal Diagnosis released a position statement in October 2021 supporting the use of NIPT for twin pregnancies as there is, “sufficient evidence showing high detection and low false positive rates with high predictive values.”¹. Our analysis of the BORN data supports this statement.

It is important to note that NIPT can sometimes fail to give a result. Based on previous publications, the failure rate in twins ranges from 1.6% to 13.2%, with a median of 3.6%¹. The failure rate appears to be higher in twin pregnancies than it is for singletons.

NIPT can be ordered through LifeLabs or Dynacare, and the requisitions can be found on our website. Please note that the MOH Criteria for Eligibility Form (page 2 of the requisition) has not yet been updated to reflect this change in criteria. Please write “twin pregnancy” under “Indication Category 1” to order OHIP-funded NIPT for your patient.  

You can use either of the provincial NIPT labs for twin pregnancies that were conceived naturally or through in vitro fertilization (IVF), where a patient's own eggs were used. Additional information on zygosity can be obtained using NIPT through LifeLabs, which can be helpful in certain circumstances (e.g. if chorionicity was not reliably established early in pregnancy).

There are two clinical situations when NIPT should be done through Dynacare, and not LifeLabs, due to differences in technology:

1. a twin pregnancy that was conceived through in vitro fertilization (IVF) using a donor egg.
2. a twin pregnancy where a gestational carrier (surrogate) is used. 

NIPT should not be done for "vanishing" twin scenarios. This is a pregnancy that started as a multiple pregnancy with subsequent loss of fetal heart activity or loss of the embryo in one of the pregnancy sacs.  NIPT is not recommended in this circumstance because there can be residual DNA in the pregnant individual's blood, from the fetus that miscarried, and this can affect the interpretation of the NIPT results for the ongoing pregnancy. 

Although usually the results are reported as either "high risk" or "low risk", the NIPT analysis can sometimes fail to provide a conclusive result. This tends to happen more frequently in twins compared to singleton pregnancies. Unlike for singletons, there is no alternative prenatal screening method available for twins. 

Follow up options:

• It would be indicated on your patient's report whether a blood redraw is an option. According to previous publications, the median success rate on redraw is about 50% (range 14.3% to 83.3%)¹. 
• Detailed anatomy ultrasound.
• Consider a referral for genetic counselling, which will include a discussion about diagnostic testing. 

Your patient is now eligible for OHIP-funded NIPT even if their FTS was screen negative (low risk). Please discuss the option of having NIPT as a follow up prenatal screen with your patient. The discussion can include balancing the benefit of having a more accurate screen with the possibility of getting a failed NIPT result, or a "high risk" result at their stage in the pregnancy. 

If your patient has not had any screening in the pregnancy, you can refer to this table to provide your patient with the chance for trisomy 21 based on maternal age / age of egg donor. This information may help facilitate decision making regarding prenatal screening.