Pregnancy tests are designed to determine if your urine or blood contains a hormone called human chorionic gonadotropin (hCG) whose in-body level is closed related with pregnancy. HCG is produced right after a fertilized egg attaches to the wall of a woman's uterus. This usually happens, but not always, about six days after fertilization. If you're pregnant, levels of hCG continue to rise rapidly, doubling every two to three days.
With obstetric ultrasonography the gestational sac sometimes can be visualized as early as four and a half weeks of gestation (approximately two and a half weeks after ovulation) and the yolk sac at about five weeks' gestation. The embryo can be observed and measured by about five and a half weeks. The heartbeat may be seen as early as six weeks, and is usually visible by seven weeks' gestation.
Haemagglutination inhibition test: The haemagglutination inhibition test was developed in the early 1960s. It used purified hCG, a sample of the woman’s urine, and anti-hCG antibodies. These substances were mixed together in a test tube. If the woman was pregnant, red cells in the mixture would clump together. The haemaglutination inhibition test was much simpler to perform than the previously used bioassays, and could be used as either a qualitative or quantitative assessment of hCG concentration
Latex agglutination inhibition test: The latex agglutination inhibition test was also developed in the early 1960s. It used polystyrene latex particles coated with hCG. A mixture of centrifuged human urine and rabbit hCG anti-serum was prepared and added to the hCG-coated latex particle suspension. If the urine was free of hCG, the latex particles would agglutinate in response to the rabbit hCG anti-serum. If hCG was present in the urine, the hCG would neutralise the rabbit hCG anti-serum and agglutination of the latex particles would not occur.
Radioimmunoassay: Radioimmunoassay used a radioactive plasma membrane receptor site containing anti-hCG antibodies to bind hCG, and in some cases to react specifically with beta-hCG. Radioimmunoassays increased the accuracy of pregnancy testing and could detect pregnancy as little as eight days after ovulation. However, they were expensive to conduct, relied on highly specialised equipment and medical personnel, and resulted in radioactive medical waste
Monoclonal antibody test: The monoclonal antibody test uses hCG antibodies, cloned from a single cell, that bind to hCG if present. The test is now commonly used as an enzyme-linked immonosorbent assay (ELISA) sandwich test involving three distinct antibodies and dye substrates that produce a colour reaction when the antibodies are activated (i.e. when a woman is pregnant). The monoclonal antibody test method is now the basis of most commercially marketed home pregnancy test kits.
