These days, a woman wanting birth control has a number of options to choose from. These various forms work in rather different ways. Today's Moment of Science addresses a form of birth control that became available in the United States in the early nineties. This form is depo-provera, otherwise known as The Shot.

Depo-provera is the brand name of a method of birth control that consists mostly of a hormone much like progesterone, a hormone that works with estrogen to regulate the menstrual cycle. What this progesterone-like chemical does to prevent pregnancy is it prevents the ovaries from releasing eggs. Without an egg, of course, sperm is rather useless. Not only this, but this hormone thickens cervical mucus in order to keep sperm from even entering the uterus to meet the egg that isn't there.

Depo-provera is one of the most effective forms of birth control. What makes it so successful is that it is so easy to administer. It requires one quite painless shot, in the hip or arm, once every three months. Oral contraceptives, on the other hand, are a bit less effective because a woman has to remember to take the pill not just every day, but at the same time everyday.

Some of the side effects of depo-provera are irregular spotting or bleeding, increased appetite and possible weight gain, sexual side effects, and depression. These side effects, however, are rare, and only affect a small percentage of women who try depo-provera. Most women who try depo-provera can expect fewer and lighter periods, though.