Pregnancy testing

Up until the 1930s the diagnosis of pregnancy was based on a combination of physical signs and symptoms such as missed periods, early morning sickness, breast tenderness and possibly a gain in weight. During the last 50 years however, scientists have developed objective tests which can detect biochemical changes in the body associated with pregnancy. In addition, the development of techniques such as ultrasound scanning now enables the embryo to be in the uterus without fear of damage.

Quite simply, if you consider that it is during the first few weeks of pregnancy that all the major organs of the body and the brain itself are being formed, you can understand why it’s important for a woman to know she’s pregnant as soon as possible.

Apart from the general value of pregnancy testing, special tests can confirm the presence of an ectopic pregnancy (one that develops in a Fallopian tube instead of the uterus). It is vitally important that this condition is detected as early as possible, as it is extremely dangerous to the woman concerned. The accurate early diagnosis of pregnancy also helps the woman with an unwanted pregnancy to take the option of having it terminated simply and effectively.

Not any longer. This used to be the case until the early 1960s. Until this time a test called a was commonly used to determine whether HCG was present in the mother’s blood or urine. Bioassays depended upon the measurement of the response of the ovaries or testes of an experimental animal (mice, rats, rabbits, frogs or toads) to an injection of HCG. These tests were not only relatively insensitive, they were also costly and time consuming, taking anything from five hours to five days to perform. Furthermore, there was the added disadvantage that large numbers of animals were required to earn’ them out With these limitations, plus the fact that simple, quick and more accurate tests are now available, there is very little reason for bioassays to be performed nowadays.

Tests called superseded bioassays during the mid-1960s, and they remain the most commonly used test of pregnancy. There are two different types- an and Immunoassay is a term used to describe any test which involves the use of to the substance being tested, and does not refer only to a pregnancy test.

Antibodies to HCG (anti-HCG) are produced by injecting an experimental animal (rabbit, sheep or goat) with HCG extracted from the blood or urine of pregnant women. Because the animal has been injected with a foreign substance its natural defence system immediately begins to fight against it by producing antibodies which are released into the bloodstream. These are then extracted (causing no harm to the animal) and used to monitor the presence of HCG in samples of blood or urine from a woman who suspects she may be pregnant.

The most common type of immunoassay is the agglutination immunoassay. This can be further divided into tube or slide tests, but the principles of both are the same. They are both quick and simple, and results can be read with the naked eye.

With the slide testadrop of anti-HCG is puton to a glass slide, and a small drop of the mother’s urine is mixed with it If the sample contains HCG it will neutralize the effect of the antibodies as the two are mixed together. If there is no HCG present to neutralize the antibodies they will remain active in the solution on the slide. In order to make the results of the test easily readable, a few drops of a milky substance made of rubber latex particles coated with HCG are added to the mixture after a one-minute interval.

If the antibodies are still present they will automatically be drawn to the HCG coating on the latex particles, and the two will clump together to form ‘curds’ in the milky substance. The binding reaction takes place within three minutes and indicates that the woman is not regnant However, if all the antibodies have een neutralized by HCG in the urine there will be no clumping together , indicating a positive result.

In the tube test red blood cells from sheep are coated with HCG and used instead of latex particles. Apart from this, the test is performed along similar lines. After mixing a small sample of urine with a known amount of an ti-HCG, the HCG coated red blood cells are added. Unlike the slide test results are not immediately detectable and the tube has to be left in a rack for two hours.

If a dark ring forms at the bottom of the tube, the woman is pregnant. The HCG in the urine sample has neutralized theanti-HCG and the HCG coated red blood cells sink to the bottom of the tube, forming a distinct ring. However, if the woman is not pregnant ho HCG is present in the urine, the HCG coated red blood cells will clump together with the antibodies and remain suspended in the solution so no ring forms.