The process of pregnancy begins with the brain signaling the ovary to release hormones on day one of the menstrual cycle. If an egg is fertilized and implanted, the levels of both progesterone and human chorionic gonadotropin, or hCG, begin to rise. hCG, also referred to as the pregnancy hormone, is what is measured by a pregnancy test.

According to Dr. James Mayer, associate professor and IVF physician at the University of South Florida College of Medicine, on day 28 of that cycle, assuming there is a healthy pregnancy in progress, the beta hCG level is usually around 100. Home pregnancy tests can pick levels as low as 25.

As for progesterone, Dr. Shayne Plosker, also an associate professor at the University of South Florida College of Medicine, says levels will rise throughout pregnancy, while hCG levels will decline. Sufficient progesterone is crucial to maintain a healthy pregnancy, simply because of the important role it plays.

"Progesterone slows down muscle contractions," Dr. Plosker says. "This is why it sometimes causes constipation, but the uterus is made of smooth muscle and it also keeps the uterus from dispelling the pregnancy."

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