Pregnancy test. How does it work? When and how is it done? Types of pregnancy test

Pregnancy test. How does it work

What determines the pregnancy test?

A pregnancy test is an immunoassay that detects a chemical in the urine or blood through an immunoassay reaction. The pregnancy test is designed to detect human chorionic gonadotropin.

Human Chorionic Gonadotropin (HCG) is a special hormone that is secreted by the embryo (embryo) coating 6-8 days after conception, after the introduction (implantation) of the embryo itself into the uterus (from this point on, the woman is considered pregnant). The name of the hormone describes exactly where it is secreted and what its function is – the hormone is secreted by the chorionic or lintular membrane of the fetal egg, and it acts as gonadotropic hormones of the pituitary gland. Gonadotropic hormones are called pituitary hormones, the action of which is directed at gonads, that is, the genital glands. They include follicle stimulating hormone (FSH) and luteinizing hormone (LH). The secretion of these hormones is naturally blocked with the onset of pregnancy, in place of hCG, which continues to regulate the hormonal background of the woman.

HCG during pregnancy performs the following functions:

Supports the function of the yellow body. Yellow body is a so-called “temporary element” in the ovaries, which acts until the 12th week of pregnancy, releasing the hormone progesterone that supports pregnancy (after 12 weeks, this function is performed by the placenta).

It has an effect on immune processes. HCG slightly depresses maternal immunity so that there is no rejection of the embryo, which is, in fact, a foreign body for maternal immune cells.

It has an impact on the uterus. HCG increases the sensitivity of uterine receptors (nerve endings) to hormones and biologically active substances. As a result, the uterus relaxes more easily, which prevents spasms and the risk of pregnancy termination. HCG also stimulates the formation of blood vessels in the uterus.

History of pregnancy tests

The fact that with the help of urine you can learn that a woman is pregnant was known back in ancient Egypt. Historical documents say that in 1350 BC women watered the seeds of wheat and barley to determine the presence of pregnancy, as well as to learn the sex of the future child. This method “worked” because of the high concentration of sex hormones in urine, so the seeds germinated.

Hippocrates suggested women to drink honey water before going to bed when their months were late. In the presence of pregnancy, a woman had spasms and abdominal pains. Less pleasant way, suggested by the same Hippocrates, was the use of bulbs. The woman placed it in her vagina for the night. In the morning, the freshness of breath was checked. If the woman’s breath smelled of onions, she was not pregnant, but the lack of smell meant that soon there would be an addition to the family.

In Europe in the Middle Ages, it was popular to mix the urine of a pregnant woman with wine. There were special “prophets” who could tell by the change of wine whether a woman was pregnant or not.
This method may also have a rational explanation, as the hormones (proteins) present in the urine of a pregnant woman could react with wine. In 20th century Europe, between 60 and 70 years old, there was a pregnancy test that was based on hormone intake and evaluation of the female body’s reaction. Pregnant and pregnant women reacted in different ways – a non-pregnant woman under the influence of hormones started a new menstrual cycle, and a pregnant woman did not react to the introduction of hormones.

Actually, it became known about hCG in 1920, when scientists found out that there is a hormone, which is produced only during pregnancy. In 20 and 30 years of the last century, to determine the presence of hCG in the urine, scientists in the laboratory introduced female urine into the bodies of frogs, mice and rabbits. In experimental animals, hCG caused changes in their genitals, which meant a positive pregnancy test result. This test was called the “A-Z test” after the doctors Asheim and Zondeck who described this method. In 1960, the first immunological method to determine hCG was used, i.e. the immune reaction of binding hCG. It is this method that is used today.

The modern pregnancy test was actively conducted closer to 1970. It was used only by doctors in specially equipped laboratories. The test was quite complicated, it took 2 hours to conduct it, so it was called “two-hour test”. In 1977, after the feminist movement in the U.S., which demanded more rights for women, the first pregnancy tests that could be carried out at home appeared in pharmacies (they were developed even earlier, but due to many factors were put on wide sale this year). The slogan of the pregnancy test in those years was “Every woman has the right to know whether she is pregnant or not”.

The author of the idea of a home pregnancy test was the designer Margaret Crane. Back in 1967, she was working on the design of cream boxes, and when she saw in the laboratory of the same pharmaceutical company as the pregnancy test, she decided to offer to create a box so that women could conduct this test at home. This test was called the “Predictor”. The home pregnancy test consisted of almost the same components that were needed to detect hCG in the laboratory. The set of such a test included a transparent bottle, a pipette, and a mirror at the bottom of the box. With the help of the pipette it was necessary to take the urine, drop a few drops into the bottle and after 2 hours observe the appearance of a red circle in the reflection at the bottom of the box. This meant that the woman was pregnant.

In 1988, a pregnancy test in the form of stripes, familiar to already modern women, was presented. And in 2003, the first digital pregnancy test appeared, which showed the result in the form of an inscription on a small screen – “pregnant” or “not pregnant”.

Pregnancy test. How does it work

How does the pregnancy test work?

The pregnancy test is based on an immune reaction. An immune reaction is the reaction of an antigen-antibody – is the process of binding an antigen, i.e. a molecule (usually a protein), to an antibody, i.e. a particle produced by immune cells to neutralize a foreign substance. The analysis is otherwise called immunochromatographic (from Greek “chromium” – color, grapho – write), because after the immune reaction a colored strip appears on the test. In pregnancy tests, the beta subunit of hCG acts as an antigen. It has room for attachment of special antibodies. The test reagent contains these antibodies, and the pregnant woman’s blood or urine is beta-hCG. Thus, if a woman is pregnant, the reaction will occur and there will be a second stripe, and if there is no beta-ChCh in the body (pregnancy has not occurred), the antibodies have nothing to bind, so the second stripe will not appear.

It is important to note that different antibodies are produced for different fragments of hCG. Different sets of pregnancy tests may have different types of antibodies. Some kits use all known antibodies to hCG, so their sensitivity, i.e. reliability is much higher, while others use only one type of antibodies, which does not exclude a false positive reaction.

When and how to do a pregnancy test?

The pregnancy test can be done 6 – 8 days after fertilization, that is, after the embryo is introduced into the uterine mucous membrane, there will be formed the lapsed envelope of the fetal egg (chorion), which will produce chorionic gonadotropin to change the hormonal background of the woman, supporting pregnancy at an early stage. Until then, the pregnancy test, even with successful fertilization and provided that the embryo is already “on the way”, will not show a positive result. The exact day of conception is not always known, so it is considered to be the day of ovulation (the day in the middle of the menstrual cycle, when the egg leaves the ovary). The pregnancy test allows to determine the beta-hCG subunit 7-10 days after ovulation, i.e. even before the delay of the period. At the same time, doctors recommend a pregnancy test a few days after the delay in menstruation. There are practically no false results at these periods.

There is another pregnancy test, it determines the early factor of pregnancy, which can be detected in the laboratory 48 hours after fertilization. This test is not widely used, and doctors do not recommend it, because successful fertilization is not a pregnancy yet. A pregnant woman will be considered after the implantation (introduction) of the fertilized egg into the uterine wall.

The pregnancy tests differ from each other in the following parameters:

  • creation date;
  • appearance;
  • method of application;
  • method of data output;
  • sensitivity.

Depending on the creation date, the next three generations of tests are distinguished:

  • 1st generation – test strips;
  • 2nd generation – test cassettes;
  • 3 generation – inkjet test.

The test of each generation differs in appearance, method of application and variant of data output. The sensitivity of the test may also vary, but it does not always depend on the test generation. Each pregnancy test is accompanied by an instruction manual. Depending on the type of test, the box may contain several test strips or one long flatbed plate, or a “pen” similar to a colored marker with a cap.

It is recommended to perform the pregnancy test in the morning using morning urine, since it is in the morning urine that the hCG content will be maximum and the accuracy of the test will be maximum. Despite this, the pregnancy test can be carried out during the day and in the evening. If the sensitivity of the test corresponds to the norm (25 mE/ml and above), the test will show the correct result at any time of day.

Pregnancy test results

Depending on the type of pregnancy test, the form of issuing the result may vary. If a normal pregnancy test is used, the data showing the result will be in the form of 1 or 2 strips on the test, depending on its result. If the test is carried out correctly, a colored strip will appear in the control area. The presence or absence of pregnancy can be found in the second zone – test zone. The most modern versions of the test use digital data, that is, on the test itself, on its small screen, there is an inscription indicating the test result.

The pregnancy test can give the following results:

  • positive – both in the control and in the test area there are stripes (the woman is pregnant);
  • negative – a strip appeared in the control zone and not in the test area (the woman is not pregnant);
  • false positive – positive test in the absence of pregnancy, if the hCG level is increased for another reason;
  • false-negative – hCG level is below the sensitivity level of the test or the test was carried out before the hCG level was raised and the pregnancy has already occurred;
  • not valid – if the strip does not appear in the control zone, it means that the test was performed incorrectly, the strip was not sufficiently soaked in urine.

The result of the digital pregnancy test may give the following options:

positive – the inscription “pregnant” or “+” appears, as well as figures indicating the number of weeks of pregnancy (1 – 2 weeks, 2 – 3 weeks, more than 3 weeks);
negative – the inscription “not pregnant” or the sign “-” appears.

Reasons for false pregnancy test results

  • Reasons for false positive results:
  • Taking medications containing hCG, such as preglue, prophase, ovitrel, choragon (other hormonal and non-hormonal drugs can not affect the test result);
  • Tumors that produce hCG (tumors of the ovary, stomach, intestines, kidneys);
  • Trofoblastic disease (tumor disease of the uterus, in which there is a growth of the lumpy layer and production of hCG after childbirth);
  • Сross-sensitivity to other hormones in the pituitary gland (if the pregnancy test determines the hCG itself and not its beta subunit);
  • The first few days after termination of pregnancy (spontaneous or artificial).

Reasons for false negative results: 

  • Early test execution;
  • Using an expired pregnancy test;
  • Urine collected in the afternoon or evening (hCG level may be lower than in the morning portion);
  • Renal disease (hCG is poorly filtered through the kidneys and does not enter the urine in the right amount);
  • Ectopic pregnancy (hCG level is low and the number of hCG increases insignificantly with the gestational age);
  • Consumption of large amounts of fluid before the test or use of diuretics (dilution of urine).

Does the pregnancy test show ectopic pregnancy?

The pregnancy test will be positive even if the embryo has not entered the uterus, but, for example, the uterus tube (ectopic pregnancy). The level of hormone hCG (human chorionic gonadotropin), which is determined by the test, however, will be lower than in a normal (uterine) pregnancy at the same period. This is explained by the fact that the uterine tube wall is much thinner than the uterine wall, and the lint of the embryo, which produces the “pregnancy hormone”, will be less pronounced.

An important point that allows you to distinguish ectopic pregnancy from uterine pregnancy is the result of repeated pregnancy test. The first pregnancy test, carried out on the days of the expected, but not occurring monthly (or slightly earlier), in both cases, may show a weak second stripe on the pregnancy test (the second stripe – a sign of pregnancy). Slightly pronounced stripe indicates a low level of hCG in the urine. In normal pregnancy, hCG level during 2 – 3 days increases 4 times, so the strip on the test becomes more pronounced. In ectopic pregnancies this does not happen because the level of hCG increases very slowly.

Can I take a pregnancy test after sex?

It makes no sense to do a pregnancy test immediately after intercourse. If conception was planned during a recent intercourse, it takes time for the sperm to reach the egg, fertilize it, and then the formed embryo reached the uterus and penetrated into its wall. Only then is the woman considered pregnant, and the test will show a positive result. Taking into account that the sexual intercourse committed in the middle of the cycle is the most likely to end with conception, it is recommended to conduct a pregnancy test 7-10 days after the day of ovulation (egg yield, which falls on day 13-15 of the menstrual cycle).

When will the test show pregnancy accurately after conception?

The test after conception will show a positive result 7 – 10 days after ovulation (middle of menstrual cycle). These are the early stages of pregnancy. At this time, not all tests can give the correct result, but only the most sensitive ones. The sensitivity of the test at an early stage should be 10 mE/ml. The best time for a pregnancy test is the days after the delay of your period. By this time, the concentration of the hormone that determines the pregnancy test becomes so high that in the presence of pregnancy it is easy to determine any sensitivity test.

When is it better to take a pregnancy test – morning, afternoon or evening?

You can perform the pregnancy test at any time of day, but the most favorable period for its implementation is the morning. In the morning portion of urine, the level of hCG (human chorionic gonadotropin), which determines the pregnancy test, is higher than in the urine during daytime and evening. This is due to the fact that during the night the urine concentrates and dilutes during the day, so if the level of hCG is still low (before the delay of the period), it is better to conduct the test in the morning hours.