In the past, predicting if an unborn baby was a boy or girl has been unscientific. But with modern technology, a baby's sex can be accurately determined by ultrasound, amniocentesis and chorionic villus sampling. These tests are used to assess the health of the developing baby and placenta and are not conducted solely to determine sex.
Prenatal ultrasound is a screening test done routinely around the first or second trimester of pregnancy. High-frequency sound waves are used to produce images of the fetus and placenta. If the baby is positioned such that the genital area can be seen, sex can be determined. The authors of a May 1999 study published in "Ultrasound in Obstetrics and Gynecology" reported 70.3 percent accuracy of predicting fetal sex using ultrasound at 11 weeks and 98.7 percent at 12 weeks.
An amniocentesis is a diagnostic test performed in some pregnancies, if there are risk factors for genetic abnormalities of the fetus. During an amniocentesis, a needle is inserted into the womb to withdraw a small amount of the fluid that surrounds the fetus. Cells from the fetus that are in the fluid are examined for genetic abnormalities. Fetal sex can be determined with near 100 percent accuracy if the appropriate tests are performed on the cells retrieved during an amniocentesis.
Chorionic Villus Sampling
Chorionic villus sampling, or CVS, involves removal of a small piece of the placenta to test for genetic abnormalities in the developing fetus. The tissue sampled -- the chorionic villi -- has the same genetic material as the fetus. Although not performed for sex determination, CVS testing does reveal whether the developing baby is a boy or girl. A hallmark report published in March 1989 in "The New England Journal of Medicine" involving 2,278 pregnant women who underwent CVS found no errors in determining the sex of the fetus.