A level Biology exam revision resources written by A level Examiners


hCG is a hormone that behaves much like LH because it stimulates the corpus luteum to secrete progesterone but has one crucial difference: it is NOT inhibited by a rising level of progesterone. So hCG prevents the deterioration of the corpus luteum at the end of the fourth week, and enables pregnancy to continue beyond the end of the normal menstrual cycle.

Because only the implanted embryo makes hCG, its early appearance in the urine of pregnant women provides the basis for the most widely used test for pregnancy (which can provide a positive signal even before menstruation would have otherwise begun).

As pregnancy continues, the placenta becomes a major source of progesterone, and its presence is essential to maintain pregnancy.

This test is usually the first test conducted when you suspect that you may be pregnant. There are a variety of home testing kits available over-the-counter and all detect the protein hormone called human chorionic gonadotropin (hCG). When an egg is fertilized, the embryo begins to produce hCG. Levels of hCG increase after conception and can be detected in the mother's urine.

Typically, the home test is a urine test for hCG:

  1. You collect a sample of urine. You would usually take the first urine in the morning, when hCG levels are the most concentrated.
  2. You can place the test stick under the stream of urine (or collect a sample and dip the wand if you prefer).
  3. The test stick has a plastic coating embedded with antibodies to hCG.
  4. The test wands also have a second antibody to hCG linked with some colour tag (e.g., coloured latex beads, or an enzyme that produces a colour reaction).
  5. If sufficient levels of hCG are present in the urine, then the hCG will bind with the second antibody and cause a colour reaction to occur (i.e., a positive test result).

As the test depends on the binding between antibodies and the HCG hormone, it is highly specific and therefore very reliable.

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