A pregnant woman can tell with between 80 percent and 90 percent accuracy that she is having a boy by having a medical professional conduct an ultrasound, if the technician can get a good view between the baby's legs, notes WebMD. Amniocentesis and chorionic villus sampling can also reveal the gender of a baby, though these tests are normally only run if doctors suspect genetic disorders or abnormalities in the fetus.
During ultrasounds, technicians use instruments called transducers, which record sound waves that develop images of the fetus, states WebMD. During the procedure, technicians apply gel to a woman's stomach to allow the transducer to capture images by gliding directly across the skin. Doctors can determine the gender of babies, due dates, the presence of multiple babies and detect problems with pregnancies using ultrasound images.
During amniocentesis, doctors remove amniotic fluid from the uterus and conduct genetic testing for conditions such as Down syndrome, and during chorionic villus sampling, they remove chorionic villi to do this testing, notes Mayo Clinic. There are several myths regarding whether women are carrying boys, including that a low-hanging belly on the expectant mother and that a pregnant woman's urine mixed with Drano turns green if the baby is a boy, notes WebMD.