Pregnancy tests are designed to tell if your urine or blood contains a hormone called human chorionic gonadotropin (hCG).

HCG (hCG) is known as the pregnancy hormone, because it is produced by the cells that form the placenta and provide nourishment to the growing embryo.  It is released when a fertilized egg attaches itself to the lining of the uterus.

Generally, a woman’s hCG levels double every 72 hours.  HCG levels reach their peak in the first 8 to 11 weeks of pregnancy and then they tend to level off for the next two trimesters.  Therefore, in the first trimester (3 months) of pregnancy symptoms can be much stronger and intense.

HCG levels vary between individual women and whether they are carrying more than one embryo.  Implantation normally occurs 6-8 days post conception followed by a very low level of hCG.  The levels of hCG increase to about 30 mIU/ml after 10 days following fertilization (conception).  During the first missed period the level is generally between 50 -250 mIU/ml.

Rapid diagnostic pregnancy tests (rapid tests) detect the presence of the hormone hCG (human Chorionic Gonadotropin) in a woman’s urine.

It is a qualitative hCG test that simply checks if hCG is present in the urine or not.  It gives a “yes” or “no” answer to the question: “Are you pregnant?”  These tests can be done at home and is also used by the doctors to confirm pregnancy.

Urine pregnancy tests detect hCG and indicate a positive result when the hCG level in the urine exceeds the sensitivity level of the test.  No test line will appear if the hCG level in the urine sample is below the sensitivity level.  This will indicate either a negative result for pregnancy or that the level of hCG is still below the detection limit of the test.

Rapid diagnostic pregnancy tests are screening tests for pregnancy and are about 99% accurate.  How accurate a home pregnancy test is depends on:

• How closely you follow instructions. Each brand of pregnancy test works slightly differently, so it is important to read the instructions before use.
• The sensitivity of the pregnancy test.
• Correct product storage and handling.
• When you ovulate in your cycle and how soon implantation occurs.
• How soon after conception you do the test.
• There are circumstances and variables that may influence a pregnancy test result from fertility medications to the diluted urine sample.

Inaccuracies are most often traced to errors in doing the test or failure to follow instructions with care.  Results may also be more accurate if you do the test first thing in the morning when your urine is more concentrated.

Home pregnancy tests (rapid tests or rapid diagnostic screening tests) are easy to use, accurate, private, convenient and the products come with instructions.  Follow them closely for the most accurate results.

If you do the pregnancy test correctly and read it within the time frame, then a faint colour test line or weak test line (in comparison to the control line) indicates a positive result and it is very likely a positive pregnancy test.

Explanations for a faint positive test line include:

• You may be testing too early after conception and your hCG level may be below the detection level of the test. Remember, hCG doubles every two days so wait and test again after 24 -48 hours using first morning urine to confirm the test result.
• The urine may be diluted due to urinating often or a high consumption of liquids. First morning urine is recommended for pregnancy testing as it contains the highest concentration of hCG.

For more information read Pregnant or not and Useful information on pregnancy tests.