It works by preventing ovulation so that an egg is not released by the ovary during your monthly cycle. With no egg to be fertilized, pregnancy cannot occur. It also causes changes in the cervical mucus that interferes with sperm reaching the egg. There are some occasions when neither of these actions prevent fertilization of the ovum (egg). Most of the time when this occurs, the fertilized egg is prevented from implanting in the uterus because the birth control hormones change the lining of the uterus so that it rejects the fertilized egg. Some might regard this as the loss of a very early pregnancy. You need to get an injection every three months or four times a year, so you don’t need to worry about taking a pill every day. If pregnancy is desired, most women can become pregnant 12 to 18 months after their last injection. It has been used around the world for almost 30 years. Many women have safely used it for more than 10 years.
You should not use Depo-Provera if you:
Although package inserts say that women who have had blood clots, heart disease and lupus should not use depo-Provera, it may be a safe alternative to combined hormonal contraception pills. If you have any of these medical conditions, discuss using Depo-Provera with your health care provider.