Chromosomes are the structures that hold genes in place. These genes determine how a baby in the womb develops. Trisomy 18 or 13 develop in about one out of 8,000 births. These trisomies cause very severe birth defects that are not compatible with long-term survival. 
Down syndrome is much more common, occurring in about one out of 800 births. Children with Down syndrome have milder physical disabilities and some degree of mental challenges. Many children with Down syndrome grow into adults who can lead rewarding lives.
Another important pair of chromosomes determines your baby’s sex. An extra or missing sex chromosome can cause syndromes like Turner syndrome (a syndrome that causes females to be shorter and have delayed puberty) or Klinefelter syndrome (a syndrome that causes low testosterone in males). 
Many parents want to know about chromosome abnormalities before birth. There are routine screening tests done to look for these and other abnormalities of fetal development during the first and second trimester. Cell-free DNA screening is more accurate than these routine screening tests and less invasive than other very accurate screening tests. [1,2]
Before cell-free DNA testing, the only option for following up on a routine screening blood test was amniocentesis or chorionic villous sampling. These procedures involve placing instruments into the womb and have potential complications that include bleeding, infection, and damage to the fetus. If a woman follows up with cell free DNA screening and the blood test comes back negative, these invasive tests can be avoided. [1,2]
How and When to Test
Cell-free DNA screening looks for fragments of DNA that show up in a mother’s blood after mixing of blood with a developing baby in the womb. These fragments appear at 10 weeks of pregnancy. The test is a simple blood test that requires no preparation. Results are available in about two weeks. [1,2]
This test is about 99 percent accurate for predicting Down syndrome and very accurate for predicting other trisomy syndromes and sex chromosome abnormalities. This test is less reliable if you are having twins or other multiple births. It is also less reliable if you are obese or if you are pregnant using a donor egg. If your test comes back negative it is highly unlikely that your baby will not have one of these syndromes. [1,2]
If your test comes back positive, you will need to have another very accurate screening test to confirm the diagnosis. This test will be a sampling of amniotic fluid or of tissue from your placenta. These tests may be done through your cervix or through your belly. [1,2]
Who Should Be Tested?
The American College of Obstetricians and Gynecologists suggest cell-free DNA screening for any pregnant woman who: 
- Is 35 years of age or older
- Has had an ultrasound or other screening blood test that suggests a chromosome abnormality
- Has a history of a previous child with a chromosome abnormality
Even if you don’t meet these requirements, you can still request cell-free DNA screening test if you want it for any reason. Weather you request the test or it is suggested, it is always a good idea to meet with a genetic counsellor first. A genetic counselor will go over the possible results, what each result might mean, and what the options are for dealing with these results. You should also check with your insurance carrier. This is an expensive test that may not be covered by all insurance policies. [1,2]