Getting pregnant on birth control is uncommon, but it’s far from impossible. The odds depend heavily on which method you use and how consistently you use it. With the pill, patch, or ring, about 7 out of 100 women will get pregnant in a typical year. With an implant or IUD, that number drops to fewer than 1 in 100.
The gap between those numbers comes down to one thing: human error. Methods that don’t require you to do anything after insertion have the lowest failure rates. Methods that depend on daily habits leave more room for mistakes.
Failure Rates by Method
Not all birth control carries the same risk. Here’s how the major methods compare in a typical year of use:
- Contraceptive implant: Less than 1 pregnancy per 100 women. This is the most effective reversible method available.
- Hormonal and copper IUDs: The copper IUD has a first-year failure rate of about 0.8 per 100 women. Hormonal IUDs perform similarly or slightly better.
- Injectable (the shot): About 4 pregnancies per 100 women with typical use.
- Pill, patch, and ring: About 7 pregnancies per 100 women with typical use. With perfect use, the rate drops below 1 in 100.
- Male condom: About 13 pregnancies per 100 women with typical use.
- Female condom: About 21 pregnancies per 100 women with typical use.
The phrase “typical use” means how real people actually use these methods, including occasional mistakes. “Perfect use” means following every instruction exactly, every single time. For the pill, that gap is enormous: over 99% effective with perfect use, but only about 91 to 93% effective in practice.
Why the Pill Fails More Often Than You’d Think
Combined birth control pills work primarily by shutting down the hormonal signals that trigger ovulation. They suppress the two key hormones your brain sends to your ovaries each month. When you take the pill consistently, your ovaries stay quiet and don’t release an egg.
The problem is the pill-free week. During those seven days of placebo pills, those suppressed hormone levels can start to climb again. If you then miss a pill or start your next pack late, that hormonal rebound can be enough to trigger ovulation. Research on women who deliberately skipped pills at specific points in their cycle confirmed that breakthrough ovulation increases when pills are missed near the beginning or end of a pack, right around that hormone-free window.
This is why the timing of missed pills matters so much. Missing one pill by less than 24 hours is low risk. You can take it as soon as you remember and continue your pack normally. But missing two or more consecutive pills (48 hours or more since your last dose) is a different situation. At that point, you need to use backup protection like condoms for seven straight days of pill-taking to reliably suppress ovulation again.
Progestin-only pills (the minipill) work differently. They rely less on stopping ovulation and more on thickening cervical mucus so sperm can’t reach an egg, along with changes to the uterine lining. That makes the timing window even tighter. Many progestin-only pills need to be taken within the same three-hour window every day to stay effective.
Medications That Lower Effectiveness
Certain drugs speed up how quickly your liver processes hormones, which can reduce the amount of contraceptive hormone circulating in your body. The most well-known culprit is rifampin, an antibiotic used to treat tuberculosis. It’s rarely prescribed, but it directly interferes with hormonal birth control.
Several seizure medications also reduce contraceptive effectiveness, including those prescribed for epilepsy, bipolar disorder, nerve pain, and migraines. Some HIV medications can interact with hormonal contraception as well, though this landscape changes as new treatments are developed.
St. John’s wort, an herbal supplement commonly taken for mood support, is another one that catches people off guard. It can lower the effectiveness of hormonal birth control through the same liver pathway.
If you take any of these, the interaction applies to pills, patches, and rings. IUDs and implants are generally not affected because they deliver hormones locally or at doses that aren’t significantly altered by liver metabolism.
Does Body Weight Affect the Odds?
This is a common concern, but the evidence is reassuring. A study comparing three-year failure rates among pill, patch, and ring users across different weight categories found no meaningful difference. Women with a BMI under 25 had an 8.4% three-year failure rate. Women with a BMI between 25 and 30 had an 11% rate. Women with a BMI over 30 had about an 8.9% rate. Those differences weren’t statistically significant. Overweight and obese women did not appear to be at increased risk of contraceptive failure with these methods.
Vomiting and Diarrhea Can Count as a Missed Dose
If you vomit or have diarrhea for 48 hours or more after taking a pill, your body may not have absorbed the hormones. The CDC recommends treating this the same way you’d treat missed pills: keep taking your pills on schedule, and use backup contraception until you’ve taken hormonal pills for seven consecutive days after symptoms resolve. A single episode of vomiting hours after taking a pill is less concerning, but prolonged stomach illness is a genuine risk factor for breakthrough pregnancy.
IUDs and Implants Are in a Different Category
Long-acting methods like IUDs and implants have failure rates so low they’re comparable to surgical sterilization. The copper IUD, for instance, has a 10-year cumulative failure rate of about 1.9 per 100 women, a number that rivals tubal ligation. The hormonal implant is slightly more effective, with a first-year failure rate well below 1%.
These methods work because they remove human error from the equation. Once placed, there’s nothing to remember, no pill to take, no patch to replace. They deliver continuous protection for three to ten years depending on the device. For someone whose primary concern is minimizing the chance of unintended pregnancy, these are the most reliable options short of permanent sterilization.
What to Do If You Think Your Birth Control Failed
If you’ve missed pills, had a medication interaction, or have another reason to suspect your birth control wasn’t working, the timeline for testing matters. A home pregnancy test is most reliable starting on the first day of your missed period. If your birth control makes your periods irregular or absent (common with hormonal methods), take a test at least 21 days after the unprotected or under-protected sex. A negative result that early isn’t always definitive. If you still suspect pregnancy after a negative test, wait a few days and test again.
Emergency contraception is an option if you catch the error quickly. It’s most effective within the first 72 hours, though some forms work up to five days after unprotected sex. The copper IUD, when inserted within five days, is the most effective form of emergency contraception available and can then serve as ongoing birth control.