For most women, prenatal genetic screening involves two blood tests - one done in the first trimester and one in the second trimester.  For some women, it may also involve a special ultrasound done in the first trimester.  This screening tells you the chance of your baby having one of these conditions:

• Down syndrome
• Trisomy 18
• Open neural tube defect (ONTD)

Although most babies are born healthy, all women have a chance of having a baby with Down syndrome, trisomy 18 or an open neural tube defect. 

There are different prenatal screens.  Which prenatal screen you will be offered depends on how far into your pregnancy you are when you first see your health care provider. It also depends on your age, your health history, and whether or not you are carrying more than one baby.

It is important to keep in mind that prenatal screening tells you the chance that your baby may have one of these conditions. It does not tell you for certain that your baby will have one of these conditions. You would need a diagnostic test to find out for sure.

Most women who have a prenatal screen get a result showing that chances are low for these conditions.

Approximately one in 20 women (5%) who have a prenatal screen will have a result that suggests their chance of having a baby with one of these conditions is high enough that they should consider diagnostic testing to find out for sure. Most women who get such a result will not have a baby with Down syndrome, trisomy 18 or an open neural tube defect.

You can also download the pamphlet "Prenatal Genetic Screening: It's your choice"