HOW DO PREGNANCY TESTS WORK?
by Tash Hughes of
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?
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
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
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.
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.
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. This Australian site sells the tests for only
$2 each, compared to $10.39 for 2 tests at Coles and
$6.28 for one test at Safeway.
Tash Hughes is
the owner of Word Constructions and assists businesses
in preparing all written documentation and web site
content. Tash also writes parenting and business articles for
inclusion in newsletter and web sites.
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