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Pregnancy tests.
The general level of the pregnancy hormone (Human Corionic Gonadotrophin - or hCG) in a non-pregnant woman is 5 mIU/ml in blood.
In a pregnant woman, it rises to 10mIU/ml about 10 days post conception, and keeps rising very rapidly each day into the pregnancy until it's in the very high thousands a week or two later.
A urine test will rely on the fact that as urine is stored in the bladder, the level of hCG equalises over time between blood and urine and so detection of hCG in urine tells the equivalent level in blood.
This system has limitations - if you test urine that has been in the body for several hours (i.e. first morning urine) it will show the blood hCG level. If you test urine that has only recently been produced and is affected by diuretics (such as alcohol or caffeine) the level in the urine will be lower than the level in the blood.
This can throw off a test in the very early stages.
Most commercial urine tests will detect an hCG level of 25mIU/ml or greater and show PREGNANT. These tests are very reliable (between 99.9% and 100% accurate) if used on the first day AFTER a missed period during a regular cycle.
There are tests on the market that will detect 10 - 15 mIU/ml, and these are sold as EARLY DETECTION tests, which may work as soon as 10 days post conception (around 4-5 days before a missed period) however - and it's a big however - they are not as accurate - used very early, they can be as unreliable as 60-70%.
There is also the possibility of a CHEMICAL pregnancy. This is where fertilisation has taken place and hCG spikes, but the egg fails to implant so there is an elevated hCG level on the days before an expected period, but the period takes place as normal (or late, or heavy).
In general, a test with a 25mIU/ml detection level, used after a missed period, on a urine sample taken from overnight urine, when no diuretics have been consumed, will be almost 100% reliable.
A second taken 48 hours later will give certainty.
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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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