Expectant parents who want their child's sex to remain a secret until birth are in the minority, said Dr. One example of this is congenital adrenal hyperplasia, a genetic condition in which baby girls may have genitals that appear more masculine than feminine, according to the Mayo Clinic. If the ring swings back and forth, the baby is believed to be a boy. For obvious reasons, it's usually easier to see if your baby is a boy. The test, usually given to moms 35 or older, involves inserting a needle into the uterus to remove a small amount of amniotic fluid. The American College of Obstetricians and Gynecologists and the American Institute of Ultrasound in Medicine issued a joint statement discouraging the use of prenatal ultrasounds for nonmedical reasons. However, he doesn't endorse so-called "bonding scans," which are also known as recreational or keepsake ultrasounds. Beyond this point, noticeable anatomical differences in the genitals can show up on the scan. One popular belief is that if a woman is carrying the baby high, she is supposedly having a girl, while carrying the baby low means it's a boy. But the common prenatal scan wasn't intended as an exam to find out the baby's gender; it was meant to image the developing fetus for other medical reasons, he said. They do so for several reasons: If you're dying to know whether to stock up on pink or blue onesies, you'll likely have to wait until you're about 18 to 20 weeks along. Another test that can look at the baby's chromosomes is called CVS chorionic villus sampling , which is performed during the first trimester to look for problems with the baby's chromosomes. Beyond ending the guessing game, there are medical reasons why mothers and fathers may want to learn the sex of their baby from an ultrasound. But mistakes can be made when determining gender because it depends on the clarity of the images and the skills of the person interpreting them. Both the egg from the mother and the sperm from the father carry sex chromosomes.