Plan B, often referred to as the morning-after pill, is a form of emergency contraception containing a high dose of the synthetic hormone levonorgestrel. This medication reduces the chance of pregnancy following unprotected intercourse or contraceptive failure. Because Plan B introduces a large hormonal surge into the body, temporary alterations to the menstrual cycle are a common and expected side effect. Understanding these potential changes helps individuals manage expectations regarding the timing and flow of their next period.
Direct Effects on Menstrual Flow
Changes in the intensity of the subsequent menstrual bleed are frequently reported after taking the emergency contraceptive pill. Plan B can make a period heavier for some individuals, though effects vary; others may find the flow is lighter. Clinical data indicate that a heavier period is a commonly reported side effect.
Breakthrough bleeding, often described as spotting, can occur in the days or weeks immediately following ingestion of the pill. This spotting is a consequence of the hormonal fluctuation, not the actual period. Approximately 16% of participants in clinical trials experienced this non-menstrual bleeding in the first week. These temporary shifts usually resolve within one cycle.
How Levonorgestrel Alters the Cycle
Plan B contains 1.5 mg of levonorgestrel, a synthetic progestin that mimics progesterone. This high dose temporarily overrides the body’s natural hormonal signaling. The primary mechanism involves inhibiting or delaying the luteinizing hormone (LH) surge, which signals the ovary to release an egg, thus lowering the risk of conception.
Levonorgestrel also influences the endometrium, the lining of the uterus. Progesterone regulates the stability of this lining throughout the cycle. The sudden introduction and subsequent withdrawal of the hormone can destabilize the endometrial tissue, leading to the irregular shedding and spotting observed shortly after taking the pill.
The timing of when the pill is taken influences the hormonal outcome. Taking the medication earlier in the cycle, during the follicular phase, is more likely to shorten the overall cycle length. This can result in an earlier and potentially heavier or lighter subsequent period.
Expected Timing of the Next Period
Plan B disrupts the sequence of events leading to ovulation, which affects the timing of the next menstruation. The period may arrive exactly on schedule, come earlier, or be delayed. It is common for the next period to start within a week of the expected date.
A delay is a frequent concern after using emergency contraception. This occurs because the pill postpones the hormonal cascade, resetting the timing of the luteal phase. If the period is delayed by more than seven days beyond the anticipated start date, a pregnancy test is recommended. Spotting immediately following the pill should be distinguished from the actual period, as spotting does not confirm effectiveness.
When Bleeding Requires Medical Consultation
While temporary changes in bleeding patterns are expected, certain symptoms require immediate medical attention. Bleeding that is extremely heavy, defined as soaking through two or more pads or tampons per hour for several consecutive hours, should be assessed. This level of blood loss may indicate an underlying issue.
Any severe or persistent abdominal pain, particularly if localized to one side, requires prompt evaluation. Severe unilateral pain can signal an ectopic pregnancy, where the fertilized egg implants outside the uterus. Consultation is also advised if irregular bleeding or spotting persists for longer than one month, or if the period remains significantly irregular for more than two cycles.