This is usually done at your first prenatal appointment to screen for certain diseases, including STDs, and determine your blood type.
Chorionic Villus Sampling (CVS)
Who it's offered to: Women who will be older than 35 at delivery; those who have an abnormal first-trimester screening test result.
When it's offered: At 10 to 12 weeks.
What it screens for: Down syndrome and some other chromosomal defects.
How it works: A small piece of tissue is taken from the placenta via a tube inserted through the cervix or a needle inserted through the uterine wall.
How effective it is: Almost 100 percent. It poses a miscarriage risk of approximately 1 percent.
What if ...? If a defect is diagnosed, you’ll need to decide whether to continue the pregnancy.
Who it's offered to: All women.
When it's offered: At 10 to 13 weeks.
What it screens for: Chromosomal defects such as Down syndrome and trisomy 18 (an often lethal defect); some cardiac defects.
How it works: Blood is drawn from the mother, and three fetal proteins are measured. These results are combined with the results of a detailed ultrasound (nuchal translucency) that measures the thickness at the back of the fetus’s neck.
How effective it is: It has an 80 percent to 85 percent detection rate for Down syndrome, with a 4 percent to 6 percent false-positive rate.
What if ...? If you have a positive screen and are not comfortable with the odds, you may opt for a diagnostic test to determine if your baby has a defect. Another option is to do an additional screen in the second trimester—but this will not give you a definitive answer, either.
Full Integrated Screen
Who it's offered to: All women (if it’s available in the area where she lives).
When it's offered: The first part is conducted at weeks 10 to 13; the second part, at weeks 14 to 20.
What it screens for: Down syndrome.
How it works: Results from the first-trimester ultrasound and blood work are combined with the results from second-trimester blood work.
How effective it is: “It has a very high detection rate for Down syndrome—85 percent to 90 percent—with a very low false-positive rate: 1 percent to 2 percent,” O’Brien says.
What if ...? If you do have a positive screen, you’ll need to decide if you’ll have amniocentesis to confirm the diagnosis (it’s too late for CVS).