Q&A: When is too soon for a home pregnancy test?

The urine pregnancy tests available at drug stores use a chemical reaction to turn a color when they detect enough of the pregnancy hormone human chorionic gonadotrophin (hCG). Every test has its own threshold of how much hCG is needed to show positive, so it's possible to be pregnant and still have the test come up negative until a certain hCG level is reached.

The hCG hormone has two molecules in it, the alpha and the beta, but the beta is the one that makes hCG unique. The alpha on it is identical to the alpha on other hormones, like thyroid hormone and the ovulation hormone. The pregnancy hormone used in today's urine pregnancy tests is the beta hCG. This is an improvement over the older tests that used to use the alpha molecule, so people might have gotten false positive tests.

The newer tests have become pretty sensitive, to the point where if you're late on a period (with usually regular cycles), then a pregnancy would show positive. The blood test is even more sensitive, but only beats out the urine test by a few days.

If you're irregular in your cycles or if you just had a coincidental delay in ovulation, the pregnancy test may take a while to show positive (two weeks or so after the delayed ovulation), even if the last menstrual period was some time ago.

If you go more than a month without your period and the tests continue to show negative, you need to visit your doctor to rule out cysts or other gynecological problems.