Introduction
Pregnancy tests, which can be purchased in a drugstore or pharmacy, are a popular alternative to a pregnancy test at a gynaecologist. Probably the best known brand name for drugstore pregnancy tests is Clearblue®. There are now not only different types of pregnancy test available under the Clearblue® brand, but also ovulation tests, which can be helpful in pregnancy planning, and contraceptives such as the Persona Contraception Monitor. Available for home use, these pregnancy and ovulation tests are available at a low price, depending on the model, and promise easy handling and fast results.
Functional principle of pregnancy tests
A pregnancy test basically makes use of the fact that when an egg is fertilised a few days later, the body produces the hormone ß-hCG. This hormone is very rarely found in the body of a non-pregnant woman. ß-hCG is excreted with the urine and can therefore be easily detected.
Pregnancy tests for home use usually have an absorbent surface on which there are enzymes that can react with the ß-hCG in the urine. If there is ß-hCG in the urine, the reading field of the pregnancy test would then show either a “+” or “pregnant” depending on the test. If there is no ß-hCG in the urine, a “-” or a “not pregnant” would appear in the reading field of the pregnancy test, depending on the test.
However, a positive pregnancy test does not always mean that a pregnancy actually exists. Tests of this kind, if the result is positive, only mean that there is ß-hCG in the urine. A regular pregnancy with implantation of the fertilised egg in the lining of the uterus is the most likely way to test positive.
However, implantation of the fertilised egg in a different location, such as a fallopian tube, twin pregnancy or even a mole of the bladder (a disturbance in the development of the embryo) would also lead to ß-hCG in the urine, i.e. to a positive pregnancy test. There are also certain diseases that are rare but can lead to the same result. After implantation, a rapid increase in the ß-hCG concentration in the urine is to be expected, which reaches its peak in the 8-10th week of pregnancy.
Safety of pregnancy tests
Clearblue® pregnancy tests are advertised as having 99% reliability from the first day of your period. The test can also be used up to 4 days before your expected period. However, during this interval, the ?-hCG concentration in the urine may not be sufficient and the Clearblue® Pregnancy Test may be negative.
In these cases it may be advisable to try another pregnancy test a few days later or to have a test done by a gynaecologist. A false positive could be due to reasons already mentioned in the previous section. Certain drugs, such as neuroleptics, benzodiazepines or drugs containing hCG can also lead to a positive test.
All articles in this series:
