The Depo shot provides pregnancy protection for 13 weeks (about 3 months) per injection. After that window, hormone levels drop below what’s needed to reliably prevent ovulation, and you’ll need your next shot to stay protected. There’s a built-in grace period if you’re running late, but the 13-week mark is your target.
The 13-Week Protection Window
Each injection delivers a synthetic form of progesterone that reaches effective levels in your bloodstream within 24 hours. Those levels plateau and hold steady for about 3 months, then start to decline. No study has found the hormone clearing to below-therapeutic levels before 12 weeks, which is why the standard schedule calls for a new shot every 13 weeks.
If you’re late for your appointment, you have up to 2 weeks of extra coverage. The CDC states that getting your next shot as late as 15 weeks after your last one still doesn’t require backup contraception. Past that 15-week mark, though, you’re no longer considered protected and should use condoms or another method until you get your next injection.
How Effective It Is Over Those 3 Months
With perfect use (getting every shot exactly on time), the Depo shot is 99% effective at preventing pregnancy. In real-world use, where people sometimes miss or delay appointments, effectiveness drops to about 96%. That means roughly 4 out of every 100 people using the shot will become pregnant in a given year. The most common reason for failure is simply waiting too long between injections.
Body weight does not appear to shorten the protection window. Evidence reviewed by the UK’s Faculty of Sexual and Reproductive Healthcare concluded that the shot’s effectiveness is not affected by weight or BMI, so higher-weight individuals don’t need more frequent dosing.
Two Versions of the Shot
There are two formulations, and both last the same amount of time. The standard version is injected into a muscle (usually the upper arm or buttock) and contains 150 mg of the active hormone. The subcutaneous version, injected just under the skin of the thigh or abdomen, contains a lower dose of 104 mg but provides the same 12 to 14 weeks of protection. Your provider can help you choose which one works better for you, but neither requires a different schedule.
How Long the Hormone Stays in Your System
Even though the shot’s contraceptive protection fades after 13 to 15 weeks, the hormone itself lingers much longer. After the 3-month plateau, levels drop over the following months but remain detectable for 7 to 9 months after a single injection. This slow clearance is why fertility doesn’t bounce back immediately when you stop getting the shot.
If you’re planning a pregnancy, expect a delay. Research published in The Lancet found that the median time to conception was about 9 months after the last injection. That figure accounts for the final shot’s 15 weeks of activity plus an additional 5.5-month wait for ovulation to resume. Some people conceive sooner, others take longer, but 9 months is a reasonable middle estimate. This is considerably slower than the return to fertility after stopping the pill or removing an IUD.
The 2-Year Guideline for Bone Density
The FDA recommends against using the Depo shot as a long-term method for more than 2 years unless other contraceptive options aren’t suitable. The reason is bone density. The shot suppresses estrogen as a side effect, and estrogen is critical for maintaining strong bones. Bone loss increases the longer you use the shot, and it may not fully reverse after you stop.
This doesn’t mean you must quit at the 2-year mark. It means the decision to continue should involve weighing the bone density risk against your other contraceptive options. People with existing risk factors for osteoporosis (family history, smoking, low calcium intake, small frame) should be especially thoughtful about long-term use. If you do continue past 2 years, your provider may recommend monitoring your bone health.
Getting the Shot After Giving Birth
The Depo shot can be given very soon after delivery, and some people receive it before leaving the hospital. The CDC and the American College of Obstetricians and Gynecologists consider it safe at any point in the postpartum period. However, the World Health Organization takes a more cautious position, recommending against use in the first 6 weeks after birth for breastfeeding individuals because the hormone may reduce milk supply during those early, critical weeks of establishing lactation.
If you’re breastfeeding and considering the shot right after delivery, it’s worth knowing that the Academy of Breastfeeding Medicine shares the WHO’s caution. Waiting until at least 6 weeks postpartum gives milk production time to stabilize before introducing a hormonal method. Once given, the shot lasts the standard 13 weeks regardless of whether you’re breastfeeding.