A prenatal paternity test is possible, but not without risk. According to the American Pregnancy Association, prenatal paternity testing is discouraged due to an increased risk of miscarriage.
There are two prenatal paternity tests: amniocentesis and chorionic villus sampling. Doctors perform amniocenteses in the second trimester; chorionic villus sampling usually occurs in the first trimester.
A fetal ultrasound measures the position of the fetus. The doctor gently inserts a long thin needle through the mother's belly and into her uterus, without touching the fetus, and withdraws about two tablespoons of amniotic fluid.
Chorionic Villus Sampling
Chorionic villus is small finger-shaped growths in the placenta that hold the same genetic material contained in a baby's cells. There are two methods to collect chorionic villus. Using an ultrasound as a guide in both methods, a thin catheter passes through the vagina and cervix into the placenta. The second method, similar to an amniocentesis, involves passing a thin needle through the belly into the placenta.
Amniocentesis and chorionic villus sampling risks include a slight chance of harm to the fetus and miscarriage. Other risks include cramping, leaking of amniotic fluid, uterine infection and vaginal bleeding.