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Human Chorionic Gonadotropin, or hCG, is also called the pregnancy hormone. It is the hormone that blood and urine pregnancy tests look for. This hCG hormone starts being produced by a rudimentary placenta when the fertilized egg implants into the uterus. Implantation occurs about six to twelve days after conception. To get a positive reading on a pregnancy test, your body must be producing hCG.
The tricky thing about predicting hCG levels is that you may be pregnant, but if implantation has not happened, hCG is not produced. In addition, home pregnancy tests may not be able to measure hCG levels in your urine until hCG has a chance to build up in your body. This can take a couple of days to a week. This is why some 'early' pregnancy tests have a high false negative rate.