What is prenatal screening?
Prenatal screening provides a woman with her own risk of having a child with:
  1. Down syndrome (also called trisomy 21)
  2. Open Spina Bifida (open neural tube defects).
Each woman starts with a risk of having a child with a chromosome abnormality (chromosomes are explained under 'Conditions Screened') based on her age at the time of delivery.  Screening adjusts this risk, either lowering or raising the risk based on the substances measured in the blood and/or measurments taken during the ultrasound.
 
It does not screen for all chromosome abnormalities, so some may be missed.  It also does not screen for every birth defect.  Prenatal screening tests cannot tell you for sure whether or not your baby has one of these three conditions, only the chance of this. To find out for sure if your baby has one of these conditions, you would have to have a diagnostic test like chorionic villus sampling (CVS) or amniocentesis.
 
Screening tests can be done starting at 11 weeks of pregnancy and are available for ALL pregnant women.

Prenatal screening tests are optional - it is your choice whether to have a prenatal screening test. (Is Prenatal Screening Right for Me?)