Pregnancy Test

A pregnancy test determined if a living embryo or fetus is present within a woman’s body. It is often used in conjunction with such presumptive signs of pregnancy as cassation of menstrual periods, morning sickness, breast tenderness, and urinary frequency. Pregnancy test generally measure a circulating hormone, human chorionic gonadotropin (HCG), produced by pregnancy tissue and present in the urine and blood of pregnant women.

Early tests were based on the injection of urine potentially containing HCG into frog or rabbits to determine any effect of HCG. Such biologic assays have been replaced by more accurate methods that measure HCG levels in urine or blood. The urine pregnancy test (immunoassay) determines if the level of HCG is consistent with known pregnancy levels and is usually positive two weeks after conception or several days after a missed menstrual period. Widely available home pregnancy tests use an immunoassay for HCG with generally accurate results. A blood test (radioimmunoassay) is a more sensitive test that can detect minute quantities of HCG. This test can be positive within days of conception and before a missed period.

Pregnancy test, in addition to diagnosing pregnancy, are also used to asses the viability of pregnancy, or the presence of tubal pregnancy. HCG can also be produced by some tumors, in which case pregnancy tests can be used as a marker of tumor activity.

With new radioimmunoassay techniques, false negative test are uncommon. The most common cause of false negative test is advanced pregnancy, because HCG values tend to decrease during the course of gestation. False positive test may occur, possibly due to certain medications, or by such clinical situations as incomplete miscarriage, ectopic pregnancy, and tumor.