The effectiveness of any birth control method is often described by two different rates: perfect use and typical use. Perfect use refers to the failure rate when a method is used exactly as directed, resulting in a very low chance of pregnancy, often less than one percent for hormonal methods. Typical use reflects real-world effectiveness, accounting for human error like missed doses or incorrect application, leading to a significantly higher failure rate, such as 7% for the pill, patch, or ring. The periods when a person is most fertile while on birth control relate directly to circumstances that shift protection from the perfect-use rate to the higher typical-use rate. These high-risk periods occur when the consistent supply of hormones is interrupted or when external factors interfere with the body’s ability to process the contraception.
User Error: The Primary Cause of Compromised Protection
The most frequent cause of a temporary high-fertility window stems from errors in the daily or weekly regimen of short-acting hormonal methods. For combined hormonal birth control, protection is maintained by keeping hormone levels stable enough to prevent ovulation. Missing a pill, patch, or ring replacement by more than 24 hours causes a drop in hormone concentration, which may allow the body’s natural ovulation process to begin.
The timing of a missed dose within the cycle determines the risk level. Missing pills during the first week of a pack is the highest risk period because a lapse in hormones can most easily allow a dominant follicle to develop and release an egg. This risk is heightened if unprotected sexual activity occurred immediately preceding the missed dose.
Missing two or more consecutive pills clearly compromises contraceptive protection, regardless of the week in the cycle. To re-establish effectiveness, the user must complete seven consecutive days of active hormone pills, known as the “7-day rule.” Until this period is complete, the user is in a high-fertility state, and backup protection is necessary. Progestin-only pills (mini-pills) are highly sensitive to timing, requiring them to be taken within a three-hour window each day to maintain their primary effect of thickening cervical mucus.
External Factors That Reduce Effectiveness
Beyond user error, certain outside influences can temporarily reduce the body’s ability to absorb or utilize synthetic contraceptive hormones, leading to a higher fertility risk.
Medications and Supplements
Some medications increase the speed at which the liver metabolizes contraceptive hormones, clearing them from the body too quickly to maintain a protective level. Specific drug classes, such as certain anti-seizure medications (like carbamazepine or phenytoin) and the antibiotic rifampin, induce liver enzymes that rapidly break down hormones. The herbal supplement St. John’s Wort operates similarly and can significantly decrease hormone concentration in the bloodstream. When taking these substances, the contraceptive method is less effective, and a backup barrier method should be used for the duration of the treatment and for seven days afterward.
Gastrointestinal Issues
Gastrointestinal distress, such as severe vomiting or diarrhea, can compromise the absorption of oral contraceptives. If vomiting occurs within two to three hours of taking a pill, the hormones may not have been fully absorbed. In this situation, the user should treat the event as a missed pill, placing them in a temporarily higher-risk window until hormone levels are stabilized.
Fertility Risks Associated with Long-Acting Methods
Long-Acting Reversible Contraceptives (LARCs), such as hormonal and copper intrauterine devices (IUDs) and implants, have extremely low typical-use failure rates because they eliminate the possibility of user error. However, a high-fertility risk occurs if the device is no longer correctly positioned or functional within the body.
Device Displacement or Expiration
For IUDs, the primary concern is expulsion, where the device partially or completely moves out of the uterus. If an IUD is fully expelled, contraceptive protection is immediately lost, and pregnancy risk returns to baseline. Users should check IUD strings regularly, as a change in string length or an inability to feel the strings may indicate displacement. A temporary high-risk period also arises if a LARC method is left in place past its approved expiration date.
Post-Removal Fertility
The period immediately following the removal of a LARC method is another time when fertility is highest. Hormonal and copper IUDs, as well as implants, do not cause a delay in the return to normal fertility. Ovulation and the ability to conceive can return immediately upon removal, often within the first menstrual cycle.
Restoring Protection and Emergency Measures
When a lapse in contraceptive use or external interference occurs, users enter a temporary high-risk window and must take immediate steps to mitigate the chance of pregnancy. The period of compromised protection lasts until seven consecutive days of correct hormonal use have been completed. During this recovery period, a non-hormonal barrier method, such as condoms, should be used for all sexual activity.
If a high-risk event, such as a missed pill in the first week or an IUD expulsion, is followed by unprotected sexual activity, emergency contraception (EC) should be considered. EC is most effective when taken as soon as possible. The two main types of EC are levonorgestrel-based pills, which have a window of up to 72 hours, and ulipristal acetate-based pills, which are effective for up to five days. The specific type of EC may require a temporary pause in the use of the regular hormonal method, as some EC can interfere with standard birth control effectiveness. Following a high-risk event, particularly if EC was used or a period is missed, a pregnancy test is appropriate. It is recommended to wait at least three weeks after the high-risk sexual activity or EC use before taking a home pregnancy test to ensure an accurate result. Consulting a healthcare provider is the best way to determine the optimal course of action for resuming full contraceptive protection.