If you’ve ever had an “oops” moment after having unprotected sex or a slip-up with your regular form of birth control, you might be familiar with Plan B, aka the “morning-after pill.”

This form of emergency contraception can be a lifesaver as a backup method for preventing pregnancy, and it’s readily available at most drugstores, with no ID required. 

We know this little pill can save you from a lot of worry, but could Plan B make your period late? Let’s start at the beginning.

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Let’s talk about periods

Most people typically get their first period sometime between ages 10-14, yet not all women experience periods. And almost half of women don’t properly understand their monthly cycles. 

Your menstrual cycle starts the first day you get your period, which, according to the Mayo Clinic, can occur every 21 to 35 days and last 2 to 7 days. However, everyone’s body is different, so a “typical” cycle is what’s typical for you. 

Throughout your cycle, hormones help your eggs mature and get your uterus ready for a potential pregnancy. Around cycle day 14, during ovulation, your ovaries release an egg, and the uterus prepares for possible implantation. If fertilization doesn’t happen, your uterine lining sheds and exits your body as your period.

Tracking your cycle, whether with a period tracking app or marking the date on your calendar, can help you keep tabs on what your body is up to.  By monitoring your periods' start and end dates, you can predict your next cycle, plan for pregnancy, or simply understand how your cycle affects your life. 

Beyond tracking your period, noting other symptoms like flow, pain, bleeding changes, or mood shifts can give you a better idea of what’s going on with your menstrual health. 

As we age, our cycles tend to regulate and shorten. Most people experience their final period between ages 45 and 55, marking the onset of menopause.

What affects your period’s timing?

It’s important to remember that menstrual cycles are unique to each person. Various factors can influence their length, amount, and accompanying symptoms, like pain or mood changes.

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Stress, dietary changes, or sudden weight fluctuations can cause irregularities in your menstrual cycle, making paying attention to your body’s signals and any changes from your usual cycle even more important. 

“There are so many different factors that can affect your menstrual cycles, combined they can have a more prominent effect,” says OB/GYN Cordelia Nwankwo, MD, FACOG. 

The Mayo Clinic lists some causes of menstrual cycle irregularities: 

  • Pregnancy or breast-feeding

  • Disordered eating

  • Extreme weight loss

  • Too much exercising

  • Polycystic Ovary Syndrome (PCOS)

  • Premature ovarian failure

  • Pelvic Inflammatory Disease (PID)

  • Uterine fibroids

Some people take hormonal birth control pills to regulate their periods. Treating the causes of certain irregularities may also help. 

Since your period plays a big role in the conception process, it’s a good idea to understand your cycle if you’re sexually active. And honestly — even if you’re not.  

If you're concerned about an unplanned pregnancy after having unprotected sex, emergency contraception options like Plan B can help.

How does Plan B work?

Plan B delays ovulation to prevent pregnancy, just like birth control pills. According to the manufacturer’s website, “When used as directed, about 7 out of every 8 women who could have gotten pregnant did not become pregnant after taking Plan B.”

The sooner you take the morning-after pill, the better it works, ideally within 72 hours of having unprotected sex. Although Plan B shouldn’t be used as your primary form of birth control, it can act as a safety net for those who want to avoid becoming pregnant. 

Can Plan B cause irregular periods?

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In short, yes. Using the pill as an emergency contraception can delay your period by up to one week, according to The Mayo Clinic. “It is fairly common for Plan B to alter your period temporarily, though not everyone will have a delay,” says Dr. Cordelia.

Similar to other medications, taking Plan B can cause side effects. “The impact should only be temporary. If you're noticing changes beyond 1-2 cycles, then you should see your doctor for evaluation,” says Dr. Cordelia.

Some of the symptoms, as listed on the manufacturer’s website, resemble symptoms experienced during early pregnancy: 

  • Lighter, heavier, early, or late periods

  • Nausea

  • Lower abdominal cramps

  • Tiredness

  • Headache or dizziness

  • Breast tenderness

  • Vomiting

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Keep an eye on your symptoms. If you’re concerned, consult your healthcare provider. Dr. Cordelia recommends taking a pregnancy test if your period is more than 1-2 weeks later than normal. 

Remember, there’s no shame in using Plan B if you have unprotected sex and want to avoid an unplanned pregnancy. It’s a safe and responsible choice. However, for ongoing protection, consider using a reliable form of birth control if you don’t want to get pregnant right now.

Regular checkups with your healthcare provider are also essential to prioritize your sexual health and well-being, ensuring both you and your partner(s) stay safe and informed.


Blair Sharp is a freelance writer who lives in Minnesota with her husband and son. Her words have been published in various publications, including Parents, SheKnows, The Bump, and Insider. Find her writing daily on LinkedIn and check out her weekly newsletter, Hey Freelancer! Head to her website www.blairsharp.com for more.