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A pregnancy test is an early diagnostic method based on the detection of specific hormonal markers in a woman’s urine. The main such marker is human chorionic gonadotropin or hCG for short. Express testing, thanks to compact and convenient devices, can be carried out at home, which is used by millions of women around the world.
HCG is a special pregnancy hormone that is produced by the cells of the membrane that covers the fetus. It is called “chorion”. Chorionic gonadotropin is present in the blood serum of both men and non-pregnant women, but its amount does not exceed 5 mU / ml. After conception occurs, the level of hCG rises sharply. By the end of the first week, it reaches 25-300 mU / ml and will continue to increase further. If, according to a blood test, pregnancy can be detected already on the 6-10th day after the fertilization of the egg, then in the urine the desired concentration of the hormone most often reaches its values on the 10-15th day. Although some test manufacturers claim that using their devices, pregnancy can be determined even on the 7th day after conception.
There are several types of rapid tests on the market that allow you to diagnose pregnancy at home.
All of them have different sensitivity to chorionic gonadotropin, ranging from 10 mIU / ml to 25 mIU / ml. Test systems include:
Strip tests or test stripsthat are impregnated with the reagent. It is an antibody to chorionic gonadotropin that reacts with it and stains the indicator in a bright color. For the study, you will need a portion of urine. If it contains a high level of hCG, then two strips will appear on the test, which indicates pregnancy. If there is no pregnancy, then the strip will be one.
Tablet Tests are a paper strip, which is enclosed in a plastic box. It has two windows. A few drops of urine are added to one of them using a pipette. The liquid reaches the reagent applied in a thin layer, and if hCG is present in the urine, then the test result will be positive. It appears in the second window and is represented by two bars. If there is no pregnancy, then only one band will appear – the control one. It is necessary in order to understand that the test is not defective and worked correctly.
Inkjet pregnancy test differs from its “brothers” in form and method of use. The system itself is represented by an elongated plastic cassette, which has a holder and a tip covered with a cap. To conduct a study, the tip of the cap is released and brought under the stream of urine. On a fibrous rod, which has tubules, urine rises up and reaches the reagent. If hCG is present in the urine, the test produces two strips (positive result). If the level of hCG in the urine is low, then the test produces 1 strip (negative result).
Electronic tests operate on the principle of inkjet tests, but the answer is displayed on the digital display that the device is equipped with. Moreover, the result is manifested in the words “pregnant” or “not pregnant”, or in the signs “+” and “-”. Such devices can additionally indicate the gestational age and even recommend the optimal date of conception if it has not yet arrived. Some modern pregnancy tests are connected to a computer via a USB port and display all the information on a monitor.
The most sensitive tests are inkjet devices with or without an electronic scoreboard, and tablet tests.
Why does a pregnancy test show a positive result?
If the pregnancy test shows a positive result, then this means that it has interacted with the hCG hormone in the urine, and the woman is pregnant. This is the most obvious reason why a pregnancy test comes back positive. However, there are exceptions to every rule. Sometimes it happens that the test is positive, and the woman is not pregnant. In this case, we can talk about a false positive result of a pregnancy test. The reasons for obtaining it will be indicated below.
Why does a pregnancy test show a negative result?
If the pregnancy test shows a negative result, then the woman is not pregnant. However, as in the case of a positive pregnancy test result, not everything is so simple.
It is possible that the test gives a false negative result, which can be obtained for a number of reasons:
The test was taken too early. Sometimes a woman, in an effort to quickly find out the result, is in a hurry to conduct a study. In this case, the answer may be negative, not because there is no pregnancy, but as a result of a low level of hCG in the urine. Another situation in which testing is carried out too early is due to the fact that a woman has an unstable menstrual schedule. In her opinion, she conducts a study on the first day of a missed period, and he has not yet arrived. And the conception itself could not occur in the middle of the cycle, but at its end. Therefore, hCG by the first day of delayed menstruation does not reach the level that could be determined using a pregnancy test.
The test can give a negative result in the actual presence of pregnancy when the urine is highly diluted. This can happen if a woman consumed a large amount of liquid during the day, and the study was performed in the evening. Reducing the concentration of hCG in the urine contributes to the intake of diuretics. Therefore, doctors recommend conducting a study in the morning, using the first portion of urine for testing.
If a woman has an ectopic or missed pregnancy, then the test for a long time may show a negative result. The fact is that during a pathological pregnancy, hCG is produced, but in smaller volumes than during a normally developing pregnancy. As a result, the test will still show two strips, but this will not happen in the early stages.
Against the background of diseases of the urinary system, the test result can be negative even with an existing pregnancy, since due to impaired renal function, hCG will not be excreted from the body in the proper volumes.
It is also not necessary to exclude such reasons for obtaining a false negative test result as a breakdown of the device, its improper storage or an expired expiration date. While the main reason for a pregnancy test showing a negative result remains the absence of pregnancy.
When does a pregnancy test give a false positive result?
Sometimes it happens that a pregnancy test shows a false positive result.
This can happen for a number of reasons, for example:
A woman in the body has a trophoblastic neoplasm that produces the hormone hCG (chorionepithelioma, choriocarcinoma). Or it can be an ovarian tumor, uterine fibroids, etc.
A false positive test result is also observed if the treated tumor recurs.
Oncological diseases such as colon cancer, stomach cancer, kidney cancer, and lung cancer can lead to a false positive result.
With brain tumors or injuries in the pituitary gland, chorionic gonadotropin can be produced in excess amounts.
Another reason for a false positive pregnancy test is a hydatidiform mole. It also belongs to the group of trophoblastic diseases and is characterized by the transformation of the villi of the outer shell of the embryo into cysts.
The test has already expired or the woman has acquired an unusable device. Sometimes tests are simply defective or stored incorrectly. Therefore, before buying, be sure to check the expiration date of the test. It will not be superfluous to ask the pharmacist for a certificate for the device. The integrity of the packaging in which the test is located must not be compromised.
During the study, the woman departed from the instructions. It is possible that the test has been exposed to urine for too long or too little. It is possible that the result was evaluated later than the time indicated in the instructions for use, etc. In order to minimize the risk of obtaining a positive result against the background of the absence of pregnancy, it is necessary to strictly follow the instructions. It is attached to each device and defines a strict sequence of actions.
The woman has recently had an abortion, miscarriage, or ectopic pregnancy. In each of the three listed cases, the level of hCG in the body will return to normal, on average, in a month. Therefore, during this time, the test result may be positive, despite the absence of an embryo in the uterine cavity.
A woman takes medications that contain human chorionic gonadotropin. They are used to treat infertility. This may be the drug Pregnil, Profazi, Ovitrel, etc. The test in this case does not make sense. Moreover, the result can be positive even after the completion of the therapeutic course, since the components of the drug are not immediately excreted from the body. This will take about two weeks.
Although there are many reasons why a test can show a false positive result, it is extremely rare for devices to make mistakes.
Will the test show a frozen pregnancy?
If a woman has a frozen pregnancy, then the test will show it. Most likely, the result of pregnancy diagnostics will be positive, but it is impossible to determine with the help of a test what kind of pregnancy a woman has.
A positive test result is not a basis for suggesting exactly how a woman develops a pregnancy – normally, or she froze. Only a doctor can help determine this issue.
As for why the test gives a positive result against the background of a missed pregnancy, the answer lies on the surface. The fact is that hCG will continue to be produced by the embryonic membrane, even if the fetus itself has already died.