Pregnancy test
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A pregnancy test is a test to determine whether or not a woman is pregnant.
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hCG measurement
All tests currently available to test for the presence of a pregnancy look for the presence of beta hCG or human chorionic gonadotropin in the blood or urine. They are usually performed after a missed menstruation or 2-3 weeks after ovulation.
This hormone is released by trophoblastic tissue in the placenta. In rare cases, it may be produced by a choriocarcinoma or some other germ cell tumours, or even other forms of cancer, i.e. lung cancer.
Doubling rule
Doctors can also perform quantitative a serum beta hCG levels. In the first two months hCG levels are supposed to double every 2-3 days. Failure to do so may indicate that the pregnancy is not developping well and can be an early sign for a possible miscarriage. Also, an ectopic pregnancy may be suspected when hCG levels fail to double. Other uses of a quantitative beta hCG reading are in evaluation of trophoblastic and other germ cell tumors.
Correlation to obstetric ultrasonography
Once a pregnancy has advanced past the first 6-8 weeks, a pregnancy is usually easier to follow by sonography as more information is obtained in real time. In general a pregnancy is detectable 25 days after ovulation by transvaginal sonography ( usally corresponding to an hCG level of > 1,500 miu/ml).
False positive/negative tests
False negative readings (from home kits) can result in early testing when the concentration of the hCG in the woman's urine is below the threshold of detection by the test. This can occur before the 5th week of pregnancy (defined as 5 weeks from the last menstrual period) - beta hCG levels rise exponentially in the first two months or so of pregnancy so the earlier the test is performed, the higher the chance of a false negative result. A blood test usually detects hCG 2 weeks after ovulation.
False positive results can result from rare beta-hCG producing tumors like choriocarcinomas. Also, most tests show a positive or unclear result after 10 to 20 minutes, independent of an actual pregnancy.
Some individuals react to some substrate in the test and thus will display a consistent low positive pregnancy test even though they are not pregnant. This phantom hCG may lead to serious misdiagnosis and intervention, but can be detected with serial dilutions.
History
The ancient Egyptians watered bags of wheat and barley with the urine of a possibly pregnant woman. Germination indicated pregnancy, and based on what type of grain sprouted they predicted the gender of the fetus. Hippocrates suggested that a woman who had missed her period should drink a solution of honey in water at bedtime. Resulting abdominal distention and cramps would indicate the presence of a pregnancy. Avicenna and many physicians after him in the middle ages performed uroscopy, a nonscientific method to evaluate urine. Selmar Aschheim and Bernhard Zondek introduced testing based on the presence of hCG in 1928. In their test an infanile female mouse was injected subcutaneously with urine of the person to be tested, and some time later the mouse was sacrificed. Presence of ovulation indicated that the urine contained hCG and meant that the person was pregnant. A similar test was developped using immature rabbits, the rabbit test. Here, too, it was necessary to kill the animal to check its ovaries. An improvement arrived with the frog test that was still used in the 1950s. A male frog was injected with serum or urine of the patient. If the frog produced sperm within the next 24 hours, the test was positive. In the frog test, the animal remained alive, and could be used again.
References
Speert H: Iconographia Gyniatrica. FA Davis Company, Philadelphia, 1973.de:Schwangerschaftstest