hCG (Human Chorionic Gonadotrophin)
This is the hormone that indicates you're pregnant. It is produced once the embryo is implanted and - in order for a positive pregnancy test - there must be sufficient levels for it to be detected. This is generally two weeks after conception (ie after the sperm has fertilised the egg, which in turn may be up to 24 hours after ovulation occurred).
Once it is produced, it causes an increase in supply of blood to the pelvic region and it is that than causes an increased desire to urinate in early pregnancy. It also ensures the endometrium doesn't disintegrate so the pregnancy can be maintained.
hCG may also be used to treat fertility problems.
For more information see:
Am I Pregnant?