A pregnancy test attempts to etermine whether or not a female is pregnant. Records of attempts at pregnancy testing have been found as far back as the ancient Greek and ancient Egyptian cultures. Modern pregnancy tests look for chemical markers associated with pregnancy. These markers are found in urine and blood, and pregnancy tests require sampling one of these substances. The first of these markers to be discovered, human chorionic gonadotropin (hCG), was discovered in 1930 to be produced by the placenta. While hCG is a reliable marker of pregnancy, it cannot be detected until after implantation: this results in false negatives if the test is performed during the very early stages of pregnancy. Obstetric ultrasonography may also be used to detect pregnancy. Obstetric ultrasonography was first practiced in the 1960s; the first home test kit for hCG was released in the mid-1970s.