How Do Pregnancy Tests Work?
All pregnancy tests today work by detecting a substance called Human Gonadotropic Hormone or
hCG. It is a bio-chemical that is made by the fertilized egg at first. This production starts even as
the egg is traveling down the tube towards the uterus, before the egg is implanted in the uterus and
there is truly a pregnancy. But, the amounts at the beginning are very small. Most pregnancy
tests are not sensitive enough to detect levels this small. Later, when the fertilized egg implants in the
uterus and the placenta forms, the placenta produces much larger amounts of hCG that can be
picked up with almost any of the pregnancy tests. However, all pregnancy tests were not created equal. Some can detect hCG at much lower levels than others and can thus detect a pregnancy earlier.
Blood Pregnancy Tests
Blood pregnancy tests (also called serum tests) can detect the hCG at much lower levels that the urine
tests. To give an idea about the difference, the blood tests can detect levels as low as 2-5 IU/L
(IU/L = International Units/liter, don't worry about what that means, this is just for comparison). Most
home urine pregnancy tests detect levels of hCG as low as 100 IU/L , but the range is 25-250 IU/L. However, blood pregnancy tests are only done for serious medical reasons, such as suspected ectopic pregnancy. As anxious as a woman may be to know if she is pregnant or not, a blood test will not be done for a routine pregnancy.
In general, the lower the level of hCG the test can detect, the earlier it can detect a pregnancy. Even
though these very sensitive blood tests can detect levels of hCG in the single digits, the standard is not
to report the result positive unless the level is 25 IU/L or above. Levels between 5-25 IU/L are
considered inconclusive. Only levels below 5 IU/L are truly negative.
The level of hCG rises steadily throughout the first ten weeks of pregnancy. It doubles about every 1.4
to 3.5 days between weeks 4 and 6. This is the time when women often suspect they are pregnant.
Given that pattern and the fact that blood tests can give an absolute number to the amount of hCG, it is
possible to repeat the test between two to four days and compare all the values. If there is a rise in the level of hCG that fits into the upward pattern expected of a normal pregnancy, then perhaps the first test was simply performed too early. When significant levels of hCG are detected, but they don't follow the normal upward path, there may be an abnormal pregnancy, an impending miscarriage, and sometimes an ectopic pregnancy. Still, the hCG levels are only one clue; the different patterns of the different conditions overlap quite a bit. It will be necessary to examine the woman, ask her questions, and do other tests to decide which one applies to her.
Urine Pregnancy Tests
Since blood pregnancy tests are only performed for serious medical reasons, they are not available to buy over-the-counter for home use. Therefore, at-home kits are all urine tests. These kits give a "yes or no" answer about pregnancy. A positive result happens whenever the level of hCG in the woman is more than the threshold amount the test is able to detect. Most home pregnancy tests are only accurate 10-14 days after a missed period (not intercourse). A positive home pregnancy test is almost always right; however, a negative test is more likely to be wrong. If the test comes out negative, the woman should repeat the test several days later to clear up any doubt. Here is a list of the urine tests that can be purchased at a local drugstore and the amount of hCG they each can detect:
a) Be Sure Plus: 25 IU/L
b) E.P.T: 40 IU/L
c) First Response: 100 IU/L
d) Clear Blue Easy: 100 IU/L
e) Conceive: 100 IU/L
f) Fact Plus: 250 IU/L