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Pregnancy tests detect the hCG hormone in urine.
Hormones are measured in units called IU. A pregnancy test generally detects hCG at a level of 25 mIU/mL in urine.
In a normal healthy non-pregnant woman, the hormone is present at 5 mIU/mL in blood. When urine stays in the bladder overnight, it reaches a hormonal equilibrium with the blood, and so testing for hCG in urine can tell you the level in blood.
In a woman who has conceived, the level of hormone rises rapidly to about 20 mIU/mL by the day of a missed period, hitting 30-50 mIU/mL a few days after the missed period and climbing rapidly in the weeks after to a level of tens or hundreds of thousands.
If an embryo is formed by fertilisation, the hormone level starts to rise, it is possible for the embryo to not develop into a foetus, and for the pregnancy not to happen. This hormone spike is called "chemical pregnancy", and can result in missed periods, or even positive test results.
It is possible also for a test to give a false negative result - if the test is taken too early in the pregnancy, or if a sample is used that is too dilute (not first morning, or after something diuretic such as coffee has been taken).
If you doubt that the test result matches the case history, you should always take another test first thing in the morning after 2 days have passed.
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Overhead, the Albatross hangs motionless upon the air,
And deep beneath the rolling waves,
In labyrinths of Coral Caves,
The Echo of a distant time
Comes willowing across the sand;
And everthing is Green and Submarine
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