What hormone is commonly tested for in pregnancy tests?

The hormone that is commonly tested for in pregnancy tests is human chorionic gonadotropin (HCG). This hormone is produced by the cells formed in the placenta shortly after the embryo attaches to the uterine lining. The presence of HCG in a woman's urine or blood is a primary indicator of pregnancy, making it the target for most at-home pregnancy tests and laboratory tests.

In practical terms, pregnancy tests are designed to detect HCG because its levels rise dramatically during the early stages of pregnancy, allowing for early detection. HCG levels can often be detected as soon as a few days after implantation, which is why pregnancy tests are marketed for use even before a missed menstrual period.

Other options listed, such as estrogen and progesterone, are hormones that play significant roles during pregnancy, but their levels fluctuate and do not provide a reliable indication of pregnancy status in the same way that HCG does. Luteinizing hormone, while important for regulating the menstrual cycle and ovulation, is not a marker of pregnancy. Therefore, HCG's specific role and timing in relation to pregnancy make it the correct answer for this question.