The pregnancy test is a tool able to ascertain (diagnose) the occurred conception (Video) in case of a delay in the onset of menstruation.
It is based on the identification of the beta fraction of a hormone, the human chorionic gonadotropin, (βhCG) in the urine or blood.
Considering that the female egg cell, after being fertilized by a spermatozoon, takes about 2 weeks to reach the uterus and implant itself inside it, βhCG can be detected very early through the pregnancy test.
The hormone, in fact, has the function of providing for the maintenance of the initial pregnancy, favoring an adequate environment for the development of the embryo.
The amount of βhCG in pregnancy doubles every two, three days and stabilizes during the second, third month.
To ascertain pregnancy (diagnosis), βhCG values can be detected approximately 10 or 14 days after conception, in the blood and urine respectively.
The pregnancy test can be performed via:
- urine analysis, it is the most used in Italy, it can be purchased for a fee at the pharmacy and performed at home independently. It allows to detect the presence of the βhCG hormone in the urine thanks to the use of specific antibodies. Most urinary tests consist of a plastic stick containing a test strip inside that dyes in the presence of the hormone. If the test is positive, lines, or other symbols, appear on the visible part of the strip.Since the tests on the market are slightly different from each other, it is important to always read the enclosed instructions to be sure that you know how to interpret the answer correctly. The result appears very quickly and is already readable after a few minutes. Some tests are more sensitive than others and therefore can be performed at any time of the day, however, generally, it is performed using the first urine of the morning because it contains the highest concentration of the hormone. You do not need to be fasting to perform the test. Sometimes, it can happen that the first test is negative even one or two days after the missed period. If pregnancy is suspected, it is advisable to repeat it a few days later. Some packs contain 2 tests just to be able to run it again if the first is negative. A positive urine test, if the instructions for its execution have been correctly followed, is 99% reliable
- blood analysis, there are two types of tests to be performed on blood which, if prescribed by the doctor, are paid for by the National Health Service (NHS):
- the test qualitative, only highlights a negative or positive result
- the test quantityMore precisely, it measures the exact amount of βhCG present in the blood. If repeated, it also allows you to follow the evolution of the early stages of pregnancy by checking the correspondence of the hormone levels with respect to the reference tables relating to the different weeks of the first trimester of pregnancy.
In general, for quantities of βhCG lower than 5 milli-International Units per milliliter (mUI / ml) the test is considered negative and pregnancy is excluded, which is, however, certain when the values exceed 25mUI / ml.
However, as analytical laboratories can use different methods, it is good to check the reference values (range) contained in the response.
Urine tests are slightly less reliable and can sometimes give so-called results false negatives (negative results while, on the other hand, the pregnancy is in progress) and, albeit very rarely, false positives (positive results when, on the other hand, pregnancy is absent).
It is important to read the instructions carefully before taking the test and, in case of a negative result, take into account that:
- the test may have been performed too early, that is to say with levels of βhCG too low to be detected: it is not always possible to establish the exact day on which ovulation and conception occurred. In case of a negative test, you can wait a few days and repeat it again; alternatively, you can contact your doctor and possibly perform the test by taking a blood sample
- the instructions in the package may not have been carried out correctly, for example, the waiting time may have been calculated incorrectly by allowing too much time to pass before checking the result
- drinking too much fluids before carrying out the test could dilute the urine and affect the levels of βhCG detectable in the sample
The very rare cases of false positives of the test can occur due to a recent abortion, an ongoing urinary infection, the presence of ovarian cysts or the use of an expired test, not properly stored or cleaned with a detergent.
Even taking some drugs can alter the test results. Promethazine, prescribed to treat the symptoms of allergic states, medicines used to treat Parkinson's disease, sleeping pills, tranquilizers, antiepileptics, diuretics and drugs for infertility treatments, such as human chorionic gonadotropin, can make results unreliable.Therefore, if you use drugs it is advisable to check on the package insert if they are molecules that may interfere with the test result.
Mayo Clinic. Getting pregnant (English)