Pregnancy Article:

How do pregnancy tests work?

By: Tash Hughes of Word Constructions

Most women know that a pregnancy test requires a urine sample, preferably first thing in the morning. They know that a colour change or new line appearing (depending on the test brand) will mean pregnancy, either wanted or not. But how do these tests work?

Beta Human Chorionic Gonadotropin Hormone, b-HCG, B-HCG or even βHCG, is released by a woman's body upon implantation of a fertilized embryo. This occurs approximately eight days after fertilization.

This hormone is responsible for encouraging the corpus luteum (or centre) of the embryo to release progesterone and oestrogen early in the pregnancy. By about 8 or 9 weeks of pregnancy, the placenta will take over this function and the βHCG levels will drop by 70 - 90 %. If high levels are detected between 15 - 22 weeks of gestation, it indicates problems with the embryo and this is the basis of the Triple Test for genetic disorders such as Downs Syndrome.

βHCG levels can be measured in International Units per Litre (IU/L) or International milliunits per millilitre (mIU/mL.) For men, the level is usually less than 3 whilst non-pregnant women have a level below 5, or 10 if she is menopausal.

There is no agreement on a particular level of βHCG representing pregnancy, as there is variation between women and foetuses, and even differences between pregnancies of single and multiple foetuses. Effectively, though, if a premenopausal woman tests for more than 5 it can be assumed that implantation has occurred.

Pregnancy tests that use a urine sample are all based on detecting higher levels of βHCG. Different tests have different sensitivities, often varying between 15 and 25 mUI/mL, and thus can be more or less effective very early in the pregnancy. Most tests in Australia have sensitivity greater than 20mIU/mL.

Tests are available which test for 20mIU/mL of βHCG and thus can be used as early as 6 days after ovulation, which is before the woman even expects to have a period. A result can be seen as soon as 40 seconds later, but clearer results require 5 minutes.

Pregnancy tests are available online or at pharmacies and supermarkets.

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