The principle of a pregnancy test lies in the ability of the strip to respond to the increase in the concentration of human chorionic gonadotropin in the blood. Normal in a woman who is not pregnant, the HCG content in the blood is minimal. It is not sufficient in order to interact with the reagent, which is impregnated with a test strip.
A few days after attachment of the embryo to the uterine wall, a woman's body begins to secrete a specific hormone - human chorionic gonadotropin. To determine the increase in its concentration in blood is possible by using blood analysis already after 7-9 days after conception. Home pregnancy test usually starts to produce a positive result from the first day of menstruation. Previously conduct Express analysis does not make sense, as the concentration of hormone in blood are low, resulting in high risk of obtaining incorrect results.
The increase in the concentration of HCG in the blood occurs about the 12th week of pregnancy. Every few days it increases in 2-3 times in comparison with the previous indicator. Then it will gradually decline. This does not mean that at some point the test will stop showing a positive result.
The content of HCG in the blood of the pregnant woman during the time quite sufficient to test showed 2 strips. When the content of the hormone reaches maximum, the indicator strip can be manifested most vividly. If you do a rapid analysis in the last weeks of pregnancy, it may be slightly paler.
Typically, the test begins to show a negative result only after some time after birth. The rate of decrease in concentration of HCG in the blood for each case is different, so in some situations, the test is positive and a few weeks after birth.