When you feel the first symptoms of pregnancy, you naturally head to the drug store to find a good pregnancy test. Have you ever wondered how they work?

Over-the-counter pregnancy tests look for the presence of the "pregnancy hormone" in your urine. The test you receive later in your doctor's office looks for the same hormone in your blood. The hormone is called human chorionic gonadotropin, or hCG for short. This hormone begins to build up just a few days after the fertilized egg is implanted in your uterine wall. The hCG will cause your placenta to produce progesterone after the implantation of the embryo, which is necessary to prevent the embryo's rejection.

Because it takes time for the hormone to build up to detectable levels, most over-the-counter pregnancy tests work best when taken a few days or a week after your missed period. Some brands claim to be accurate sooner, but all women ovulate at different times of their cycles, so this may or may not be true for you.

The most accurate test is done in your doctor's office. A blood test can detect much lower levels of the pregnancy hormone than urine tests. If you get a negative reading from your drug store pregnancy test, but you still "feel" pregnant, you'll want to make an appointment with your doctor. Planned Parenthood offices also offer pregnancy tests and exams.

About 25 days after an egg is fertilized the fetus can be seen by transvaginal sonography.

What causes false negative test results?

A false positive result may occur if you take your test too early. The pregnancy hormone hCG does not begin to build up in your system until after the fertilized egg has attached to the uterine wall. This usually occurs within 14 days of fertilization, so a blood test is almost always accurate if it's done after your next period was supposed to start. Some over-the-counter tests need higher levels of the hormone to be accurate, and so they say you're not pregnant, even though you are. You can take another test a few days later to be sure.

Can you get a false positive result?

Yes, your test may say you're pregnant even though you're not. The most common cause is waiting too long to read the results of the test - the urine will evaporate from the test strip, and the color will change to show a positive result. If you get a positive result but you waited longer than suggested by the test's instructions, you'll want to take the test over.

If you're taking fertility hormones, a home pregnancy test may show a false positive result. Ask your doctor if your injections include the hormone hCG. If so, the hormone will show up on the test, and the reading will be inaccurate.

In very rare instances, a false positive reading is caused by certain unusual forms of cancer.

What about the rabbit?

In your grandmother's day, there were no over-the-counter pregnancy tests. The doctor would take a urine sample, and inject the urine into a female rabbit. If the rabbit was injected with humane urine that contained the pregnancy hormone, the rabbit's ovaries would change. It usually took several days for the changes to take place, and the rabbit had to die before the lab technicians could look at her ovaries.

Fortunately, rabbits are no longer necessary for pregnancy tests.