The placebo week in combination birth control pills refers to the short period, typically seven days or less, at the end of a pill pack. Inactive pills are taken during this time. These pills do not contain hormones and serve as reminders to maintain the daily habit, allowing for a withdrawal bleed that mimics a menstrual period before starting a new pack of active pills.
How Combination Birth Control Pills Prevent Pregnancy
Combination birth control pills prevent pregnancy by introducing synthetic estrogen and progestin. These hormones work through three main mechanisms. They suppress ovulation, meaning the ovaries do not release an egg each month. Consistent hormone levels in active pills prevent the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which are necessary for egg maturation and release.
Beyond ovulation suppression, these pills also thicken the cervical mucus, creating a barrier that makes it difficult for sperm to travel through the cervix and reach an egg. Additionally, the hormones can thin the lining of the uterus. This thinner uterine lining makes it challenging for a fertilized egg to implant. These combined effects provide continuous protection against pregnancy when the active pills are taken as directed.
Pregnancy Risk During the Placebo Week
If active combination birth control pills were taken correctly and consistently in the weeks leading up to the placebo week, the protective hormonal effects generally remain. The suppression of ovulation and changes in cervical mucus and uterine lining from the active pills carry over into the placebo week. This means the risk of pregnancy during this hormone-free interval is very low, as long as active pills were not missed or taken incorrectly beforehand.
Placebo pills do not contain hormones and offer no direct pregnancy prevention. Their role is to help maintain the daily pill-taking routine and signal the scheduled withdrawal bleed. Continuous protection relies on the consistent use of active pills preceding the placebo week.
Factors That Reduce Pill Effectiveness
Several factors can compromise the effectiveness of combination birth control pills, increasing pregnancy risk. Missing active pills is a common reason for reduced effectiveness, particularly if multiple pills are missed or if a new pack is started after a break lasting nine or more days. Taking a pill more than 48 hours after the last one can also affect contraceptive coverage.
Gastrointestinal issues like vomiting or severe diarrhea shortly after taking an active pill can prevent hormone absorption, potentially reducing effectiveness. Certain medications and herbal supplements can also interfere with how the pill works. Examples include the antibiotic rifampicin, some anticonvulsants, and St. John’s Wort. Improper storage, such as exposing pills to direct light, heat, or moisture, may also compromise their effectiveness.
Actions to Take If Effectiveness is Compromised
If you suspect your birth control pill’s effectiveness has been compromised, consult the patient information leaflet included with your pill pack. It provides specific instructions for missed pills or other scenarios. If you have missed one active pill, take it as soon as you remember, even if it means taking two pills in one day, and generally no additional contraceptive protection is needed.
If two or more active pills were missed, or a new pack started two or more days late, take the most recently missed pill and continue with the rest of the pack. Use a backup birth control method like condoms for at least seven consecutive days. If unprotected sex occurred within the last five days and pills were missed, consider emergency contraception. Contact a healthcare provider for personalized advice. If a period is missed after the placebo week, particularly if effectiveness was compromised, take a pregnancy test.