According to the American Pregnancy Association, an ultrasound can be used to determine gestational age and therefore date of conception, but it is not the only procedure in doing so. It is used as a supplemental method when combined with measuring hormone levels and date of last menstruation.
An ultrasound exam, as explained by the American Pregnancy Association, uses high-frequency sound waves to map a woman's abdomen and pelvic cavity. The resulting image, a sonogram, shows the developing baby and placenta. A specialized gel is placed on the woman's abdomen in order to better conduct sound waves. A transducer is then used to project sound waves into the uterus where the sound waves bounce back, and then return, producing an image.