The Depo-Provera birth control shot is a widely used and effective method of contraception. It offers a convenient, long-acting reversible contraceptive option for many individuals, providing pregnancy prevention for an extended period and eliminating the need for daily pills or other frequent actions.
How the Shot Works
The Depo-Provera shot contains medroxyprogesterone acetate (DMPA), a synthetic form of the hormone progestin. This progestin works primarily by inhibiting the secretion of gonadotropins from the pituitary gland, preventing follicular maturation and ovulation. An egg is not released from the ovaries.
Beyond preventing ovulation, the progestin in the shot also thickens the cervical mucus, creating a barrier that makes it difficult for sperm to travel through the cervix and reach an egg. Additionally, it thins the lining of the uterus, making it less receptive to a fertilized egg. These combined actions provide a multi-faceted approach to pregnancy prevention.
Real-World Effectiveness Rates
The Depo-Provera shot is highly effective at preventing pregnancy. When used precisely as directed (“perfect use”), its effectiveness rate is around 99%. This means fewer than 1 out of 100 people using it perfectly would become pregnant in a year. Perfect use involves receiving the injection consistently every 13 weeks.
In real-world scenarios, which account for occasional delays or missed injections, the “typical use” effectiveness rate for Depo-Provera is about 94%. This suggests that approximately 4 to 6 out of 100 users might become pregnant annually under typical conditions. The Pearl Index, a common measure for contraceptive effectiveness, quantifies the number of unintended pregnancies per 100 woman-years of exposure. A lower Pearl Index indicates higher effectiveness.
Factors That Impact Efficacy
Maintaining the effectiveness of the Depo-Provera shot depends on receiving injections on a consistent schedule. The shot provides contraceptive protection for approximately 12 to 14 weeks, requiring a new injection every 12 to 13 weeks. Missing or significantly delaying an injection can reduce its effectiveness and increase the risk of unintended pregnancy.
If an injection is more than two weeks late, a healthcare provider will require a pregnancy test before administering the next dose. Backup contraception, like condoms, should be used for at least seven days after the delayed injection. Certain medications and herbal products can also reduce the shot’s effectiveness.
What Happens After Stopping the Shot
After discontinuing the Depo-Provera shot, its contraceptive effects do not immediately cease. The synthetic progestin remains in the body for an extended period, meaning fertility does not return instantly. The median time for fertility to return after the last injection is around 10 months.
While some individuals may experience a return to ovulation and regular menstrual cycles sooner, it can take up to 18 months or even longer for others to become pregnant. The duration of use does not affect how long it takes for the hormones to leave the system or for fertility to return.