Factors that can affect a prenatal screening result


This update contains information about three aspects of prenatal screening – maternal weight, maternal smoking and open neural tube defects results.

1. Maternal weight - A difference of only 5 pounds matters!

Maternal weight is an important factor for prenatal screening for Down syndrome, trisomy 18 and open neural tube defects (NTD). A difference of only 5 pounds between the actual weight and the weight used for calculating the result, has a significant impact on the accuracy of the patient’s screening result.

To ensure the most accurate results, for Integrated Prenatal Screening (IPS), First Trimester Screening (FTS), or for Serum Integrated Prenatal Screening (SIPS), please record the exact measured weight done as close as possible to the day when  the 1st trimester blood sample is taken.

For second trimester Maternal Serum Screening (MSS) or Maternal Serum alpha -fetoprotein (MSAFP), please record the exact measured weight as close as possible to the day when the sample is taken.

2. Maternal smoking - Maternal smoking influences the screening results!

 Maternal smoking changes the value of the biochemical markers used to calculate the risk of Down syndrome and trisomy 18.  If a woman who smokes does not have her status as a smoker indicated on the requisition, her risk of a false positive result is higher compared to that of non-smokers.

To ensure the most effective and accurate screen for your patients, fill in the requisition section on smoking, using the following guidelines.

Enter “Yes” on the requisition’s spot for smoking if any of the following smoking patterns is reported:

  • Smoking, occasionally in this pregnancy
  • Stopped when the first menstrual cycle was missed
  • Stopped when the pregnancy was confirmed

Enter “NO” if second hand smoke exposure is reported.

  1. NOTE:

    If your copy of the requisition does not have a box to enter smoking status, call your regional genetics centre or prenatal screening laboratory to ask for an updated version. Discard versions without smoking status on the requisition.  Also, if the version of the requisition in your office predates 2009, please ask for the new version.

3. Open neural tube defect results – change in result format

The majority of prenatal screening results no longer report the numerical risk of an open neural tube defect (spina bifida and anencephaly). The result will be reported as either screen positive or screen negative. 

The software program used by the majority of the Ontario prenatal screening labs no longer accurately reflects the true risks of an open neural tube defect because of the significant drop in the prevalence of neural tube defects attributable to folic acid supplementation.  The software program used by London Health Sciences, Children’s Hospital of Eastern Ontario, North York General Hospital and Credit Valley Hospital significantly overestimates the risk and therefore, the program has opted to not print the risk value.

Typically, a screen positive test result is telephoned or faxed directly to the provider, and in almost all cases, a level II fetal ultrasound is suggested to assess the risk of a fetal anomaly.  Thus, not printing the risk does not significantly alter patient care.

Results from the laboratory at Mt. Sinai Hospital, which uses a different software program, continue to have a risk printed on the report because the risks calculated by this software program are more reflective of the Ontario population. Longer term solutions to this disparity are being sought, but this will take some time to complete.