Pregnancy tests

A pregnancy test attempts to determine whether or not a female is pregnant. The most common tests use markers found in blood and urine, specifically one called human chorionic gonadotropin (hCG). Identified in the early 20th century, hCG rises quickly in the first few weeks of pregnancy, peaking at 10 weeks. It is produced by the syncytiotrophoblast cells of the fertilised ova (eggs) as the cells invade the uterus' lining and start forming what will become placenta. A pregnancy test attempts to determine whether or not a female is pregnant. The most common tests use markers found in blood and urine, specifically one called human chorionic gonadotropin (hCG). Identified in the early 20th century, hCG rises quickly in the first few weeks of pregnancy, peaking at 10 weeks. It is produced by the syncytiotrophoblast cells of the fertilised ova (eggs) as the cells invade the uterus' lining and start forming what will become placenta. Urine tests will typically show positive around four weeks after the last menstrual period (LMP) and are best done in the morning as hCG levels are then highest. Because of their cut-off hCG level, a positive result is less likely to be incorrect than a negative one, and how much water/fluids have been consumed can affect the results as well. Serum hCG tests for a more specific part of the hCG molecule and can detect pregnancy earlier than urine, before even a period has been missed. 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 invented in 1968. The kits went on the market in the United States and Europe in the mid-1970s. Chemical tests for pregnancy look for the presence of the beta subunit of human chorionic gonadotropin (hCG) in the blood or urine. For a qualitative test (yes/no results only), the thresholds for a positive test are generally determined by an hCG cut-off where at least 95% of pregnant women would get a positive result on the day of their first missed period. hCG can be detected in urine or blood after implantation around six to twelve days after fertilization, although some evidence suggests that hCG may be released preimplantation as well. Quantitative blood (serum beta) tests can detect hCG levels as low as 1 mIU/mL, and typically clinicians will call a positive pregnancy test at 5mIU/mL. Urine test strips have published detection thresholds of 10 mIU/mL to 100 mIU/mL, depending on the brand. Most home pregnancy tests are based on lateral-flow technology. 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. Although all current pregnancy tests detect the presence of beta hCG, research has identified at least one other possible marker that may appear earlier and exclusively during pregnancy. For example, early pregnancy factor (EPF) can be detected in blood within 48 hours of fertilization, rather than after implantation. That said, its reliable use as a pregnancy test remains unclear as studies have shown its presence in physiological situations besides pregnancy, and its application to humans remains limited. A systematic review published in 1998 showed that home pregnancy test kits, when used by experienced technicians, are almost as accurate as professional laboratory testing (97.4%). When used by consumers, however, the accuracy fell to 75%: the review authors noted that many users misunderstood or failed to follow the instructions included in the kits. Improper usage may cause both false negatives and false positives. False negative readings can occur when testing is done too early. Quantitative blood tests and the most sensitive urine tests usually begin to detect hCG shortly after implantation, which can occur anywhere from 6 to 12 days after ovulation. hCG levels continue to rise through the first 20 weeks of pregnancy, so the chances of false negative test results diminish with time (gestation age). Less sensitive urine tests and qualitative blood tests may not detect pregnancy until three or four days after implantation. Menstruation occurs on average 14 days after ovulation, so the likelihood of a false negative is low once a menstrual period is late. Ovulation may not occur at a predictable time in the menstrual cycle, however. A number of factors may cause an unexpectedly early or late ovulation, even for women with a history of regular menstrual cycles. Using ovulation predictor kits (OPKs), or charting the fertility signs of cervical mucus or basal body temperature give a more accurate idea of when to test than day-counting alone.