It is recommended that screening for aneuploidy (chromosomal abnormalities) is offered to all women during their pregnancy regardless of age. All women should be offered screening by the second trimester that includes a survey ultrasound of the fetus and hormone markers measured from the mother's blood. In many areas in the U.S., even earlier screening between, 10 and 14 weeks gestation, is offered (often called a first look or early risk assessment) that involves an ultrasound to measure of what is called the nuchal lucency (a clear space near the back of the fetuses neck) as well as measuring hormone markers in the mother's blood.
It is important to remember this form of screening, is just that, screening. It offers a risk assessment in relation to a patient's baseline risk based upon their age. This type of screening can detect up to 85% of chromosomally abnormal pregnancies with a false positive rate of ~5%, but this still has to be confirmed by chromosomal analysis.
Determing an abnormal screening test is in fact a true positive (meaning the screening test is positive which is then confirmed by an accurate diagnostic test) is done in two ways: an amniocentesis or chorionic villus sampling.
So simply stated, the blood test predicts a risk for an abnormality that then must be confirmed by a diagnostic test.
I hope that is helpful.