How do doctors tell if fetus is a boy or girl?

Couple of ways: If a mother has a genetic test like chorionic villi sampling, or amniocentesis, the lab will be able to determine whether the baby is a boy or girl. Ultrasound can also be used, but is not quite as accurate depending on the position of the baby, and size of the mother.

Simple anatomy: Most look to an ultrasound at 18 wks or so by a talented technician.For this to work the technician must detect the boys penis in its proper position. If the penis is not found, they assume it is a girl, which leads to potential for error.