The Depo-Provera shot works immediately if you get it within the first seven days of your period. If you get it at any other point in your cycle, it takes seven days to become effective. That seven-day window is the key number to remember, and the timing of your injection determines which scenario applies to you.
When Protection Starts
The shot works by delivering a synthetic form of progesterone that stops your ovaries from releasing eggs. It also thickens cervical mucus, making it harder for sperm to reach an egg. How quickly these changes kick in depends entirely on where you are in your menstrual cycle when you get the injection.
If you receive the shot during the first seven days after your period starts, you’re protected right away. Your body is already in a phase where ovulation is far off, and the hormone has enough time to suppress the process before it begins. No backup birth control is needed.
If you get the shot after day seven of your cycle, you need to use condoms or avoid sex for the next seven days. Research shows why this matters: in a study of women who received the shot between days 8 and 13 of their cycle, 30% still ovulated. Every one of those ovulations happened in women who got the injection on day 10 or later. After seven days, the hormone has suppressed ovulation reliably enough to provide full protection.
Timing After Pregnancy or Abortion
The rules shift slightly if you’ve recently given birth or had an abortion. If you get the shot within three weeks of delivering a baby, you’re protected immediately, no backup method required. You can receive the injection right after giving birth, whether or not you’re breastfeeding.
After a miscarriage or abortion, the window for immediate protection is seven days. Get the shot within that timeframe and you’re covered. If more than seven days have passed, the standard rule applies: use backup contraception for seven days after the injection.
Switching From Another Method
If you’re switching from the pill, patch, or ring to the Depo shot, timing it during the first seven days of your period gives you seamless, immediate protection. If you switch at any other point in your cycle, use a backup method for seven days. The simplest approach is to get the injection while you’re still using your current method, then let the seven-day overlap cover the transition.
How Long Each Shot Lasts
Once the shot is fully effective, a single injection suppresses ovulation for about 13 weeks. Research confirms that ovulation stays suppressed for at least 112 days (about 16 weeks) in roughly 96% of women, which builds in a small buffer beyond the standard 13-week schedule.
For follow-up injections, you have a window between weeks 11 and 15 to get your next shot without losing protection. That means you can be up to two weeks early or two weeks late and still remain covered. If you go past that 15-week mark, you’ll need a pregnancy test before your next injection, and you should use backup contraception for seven days after receiving it, just like a first-time shot given mid-cycle.
How Effective the Shot Is
With perfect use, meaning you get every injection on schedule, the shot has a 0.2% failure rate in the first year. That makes it one of the most effective reversible contraceptives available. In typical use, which accounts for people who are sometimes late for their appointments, the failure rate rises to about 6%. The gap between those numbers is almost entirely explained by missed or delayed injections.
How Long Fertility Takes to Return
The shot’s effects don’t stop the moment it wears off. After your last injection, you can expect a median delay of about 9 months before conception. That breaks down to roughly 15 weeks for the final dose to clear your system, plus another 5 to 6 months for ovulation to return to a regular pattern. Some women conceive sooner, others take longer, but 9 months is the midpoint. If you’re planning a pregnancy in the near future, this longer timeline is worth factoring into your decision.