Though birth control pills are highly effective in protecting against pregnancy, they are not a foolproof method of contraception. Here’s the key: In laboratory studies (aka “perfect use”), birth control pills are considered 99% effective. When used by actual humans (aka “typical use”), they’re more like 91% effective. That’s a failure rate of 9%, or 9 out of 100 users who may get pregnant each year.

Why? There are certain factors, such as missing a pill or taking additional medications, that can reduce the effectiveness of the pill and potentially increase your chances of getting pregnant.

If you’re trying to prevent a pregnancy and taking oral contraception, here’s what you need to know about effective prevention.

Factors that can reduce the effectiveness of birth control

While the pill is designed to prevent you from ovulating, missing a dose of the pill or forgetting to start a new pack are the main reasons why the pill won’t work as described, as the pill is intended to maintain a consistent level of hormones circulating in your body at any time.

If you take the combination pill (with the hormones estradiol and progestin), you don’t have to take your pill at the exact same time every day, but taking your dosage around the same time daily helps establish consistency and makes it more of a habit—and easier to remember. 

It should be noted that if you take the mini pill (with the hormone progestin only), you should prioritize taking the pill at the same time every day. A delayed or missed dose can cause your hormone levels to drop quickly, potentially resulting in ovulation, which means there’s a chance you could get pregnant if you have sex around that time. 

Additional factors that may impact the combination pill and mini pill’s effectiveness:

  • Vomiting or having diarrhea for more than 48 hours
  • Regularly eating grapefruit or drinking grapefruit juice
  • Taking medicines used to treat diarrhea
  • Using laxatives
  • Taking the antibiotic Rifampin, which is used to treat tuberculosis and other infections
  • Taking the antifungal Griseofulvin, which is used to treat skin infections
  • Certain HIV medications
  • Certain anti-seizure medicines
  • Taking the herb St. John’s Wort, an herbal supplement used to treat depression

If you’re taking any of these medications or have been vomiting or experiencing diarrhea for more than 2 days, which can affect the absorption of the hormones into your system, use condoms as a backup method of contraception. 

If you’ve missed a day of the pill, take the next pill as soon as you remember. You’ll need to use condoms for up to a week after you’ve missed a pill. 

Additionally, talk to your doctor about any other medications you may be taking and whether they can impact the pill’s effectiveness. 

How to prevent birth control failure

If you’re hoping to prevent pregnancy while taking the birth control pill, be sure to follow these tips: 

  • Set reminders: Create a reminder on your phone or smartwatch to ping when it’s time to take your pill, and keep your pack somewhere you’re likely to see it each day. Ideally, you’ll take it at the same time every day, like when you brush your teeth in the morning or right after dinner.
  • Take the placebo pills: Taking the inactive (placebo) pills can help you keep up the habit, even though they’re not medically necessary. It’ll help ensure you’re not late when starting your next pill pack.
  • Track your cycle: Even though you’re technically not ovulating and don’t have a traditional menstrual bleed (just a “pill bleed”, which is what happens when you take the placebo pills and go through a brief hormone withdrawal), it can be helpful to track your cycle in an app, which can help you pinpoint your pill bleed week and also give you reminders to take your pill and start your new pack. We like Clue or Spot On

Wondering if you can have sex during your placebo week without risking pregnancy? You’re still protected from conception during your pill bleed week as long as you’ve been taking the active pill doses consistently for the previous 3 weeks. 

Early pregnancy symptoms

Remember that the hallmark sign of pregnancy is a missed period, but because while taking the pill you won’t have a true period, this won’t be a helpful sign. Your first step should be to take a home pregnancy test, which can pick up levels of hormones in the blood. 

If you're on the pill but worried you might be pregnant, here are a few early warning signs to look out for:

  • Sore or tender breasts and nipples
  • A new aversion to certain foods and smells
  • Strange food cravings
  • Nausea
  • Vomiting
  • Fatigue
  • Lightheadedness

Can taking birth control pills harm your baby?

While it’s important to stop taking the pill as soon as you suspect you might be pregnant, taking the pill during early pregnancy shouldn’t harm the developing fetus.

Some scientific studies have shown a possible link between use of birth control pills at the time of conception and an increased risk of low birth weight or preterm birth or urinary tract abnormalities, but these issues haven’t been observed in clinical experience, according to Mayo Clinic.

The risk to the developing baby is generally very low, but talk to your doctor if you have concerns.