Birth control pills have a high success rate. However, they may fail and you can still get pregnant. Certain factors may increase your chance of getting pregnant while on the pill, such as missing a dose.

Birth control pills are 99 percent effective with “perfect use,” which means taking the pill at the same time every day without missing a dose. “Typical use” is how most women take the pill, and then it’s about 91 percent effective. Both combined oral contraceptives and progestin-only pills (also known as the mini pill) have a typical failure rate of 9 percent.

Many women accidentally miss a dose or forget to start a new pack of pills. When that happens, the chances for an accidental pregnancy go up.

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Certain conditions or behaviors can increase the likelihood that your birth control won’t be as effective at preventing pregnancy.

If you can’t remember to take your pill at the same time every day, you increase your risk of pregnancy. Birth control pills are designed to maintain a constant level of hormones in your body. If you skip or miss a dose, you hormone levels can drop quickly. Depending on where you are in your cycle, this may cause you to ovulate. Ovulation can increase your chances of becoming pregnant.

Reckless alcohol consumption can also cause birth control failure. While under the influence, some women may forget to take their pill at the correct time. If you vomit too soon after taking your pill, your body may not be able to absorb any of the hormones. This can result in a drop in your hormone levels, which could trigger ovulation.

Taking another medication or supplement at the same time as your birth control pill can also affect the pill’s effectiveness.

Keep these tips in mind if you’re on birth control and want to prevent pregnancy.

Time it right

Make sure you take your birth control pill at the same time every day. Set a reminder on your phone or watch if you need to. You may also consider taking the pill with a specific daily activity, such as during lunch or dinner.

If you take progestin-only pills, you should be especially careful about taking the pill at the same time every day. If you’re late with a dose or skip one altogether, your hormone levels can drop very quickly. This could cause you to ovulate and that greatly increases your chances for getting pregnant.

If you miss a dose, use a backup method or avoid sex for the next week. To be extra cautious, use a backup method, such as a condom, or avoid sex for the next month.

Take the placebo pills

Combination pill packs typically contain three weeks of active pills that contain hormones and one week of inactive, or placebo, pills. Although it isn’t medically necessary to take the placebo pills, doing so can help you stay in your routine.

If you choose to skip the placebo pills, there’s a chance that you may be late in starting your next pill pack. This can interrupt your body’s expected level of hormones and cause you to ovulate. Ovulation increases your chances of being pregnant.

Don’t mix medications

Some prescription and over-the-counter medications may interfere with your birth control’s effectiveness. Before you begin taking a new medicine, ask your doctor or pharmacist if you should use a backup method of protection while taking this new medication.

Although some antibiotics have been anecdotally connected to unplanned pregnancies, research has largely debunked this connection. A possible connection with reduced birth control effectiveness is only recognized with one type of uncommon antibiotic called rifampin.

Don’t use St. John’s wort

St. John’s wort is a popular over-the-counter herbal supplement that can affect liver metabolism. This supplement can interfere with birth control’s effectiveness. You could experience breakthrough bleeding and possibly an unplanned pregnancy if you take the two medicines together. Talk with your doctor about any additional measures you should take, including a backup protection method while you’re taking St. John’s wort.

Knowing what can make your birth control ineffective and how you can increase your chances for successfully avoiding pregnancy will help you make the best decisions for yourself.

The earliest symptoms of pregnancy can be easily overlooked, especially if you’re on birth control. If you experience any of these symptoms, take a pregnancy test to confirm your pregnancy status. If you want to double check the at-home pregnancy test, a simple blood test by your doctor can confirm your status.

The early signs of pregnancy include:

Morning sickness

Nausea, vomiting, and fatigue are also signs of early pregnancy. Contrary to its name, morning sickness can occur at any time of the day. It can begin very early after conception. While your body adjusts to the new pregnancy, you may also find yourself growing tired more easily or more quickly.

Missed period

Many women begin suspecting they’re pregnant when they miss a period. Unfortunately, some women don’t have a period while on birth control, so a missed period may not necessarily be an easy indicator.

Implementation bleeding, which happens when a fertilized egg attaches to your uterus, can be mistaken for a period. This is especially true if your period is typically very light.

If you discover that you’re pregnant, you should speak with your doctor as soon as possible. If you plan to keep the pregnancy, you’ll need to start caring for your growing baby. This means going off of the birth control pill and beginning to take a daily prenatal vitamin with at least 400 micrograms of folic acid. You’ll also have to begin preparing for an upcoming delivery.

If you decide you’d like to terminate the pregnancy, you should begin this process as quickly as you can. Depending on where you live, legal restrictions could prevent you from having the procedure after you’ve reached a certain point in the pregnancy.

Can birth control harm your baby?

You may also worry that taking birth control pills could have harmed your developing baby. According to the Mayo Clinic, this doesn’t appear to the case. Some research showed a link between birth control during early pregnancy with issues low birth weight, abnormalities to the urinary tract, and preterm delivery, but little has been observed clinically. It’s important to stop taking the pill as soon as you suspect pregnancy, but your baby shouldn’t be at a greater risk of defects.

If it’s taken correctly, birth control is a wonderful method for preventing pregnancy and helping with a number of other conditions, including acne and painful menstrual cramps. As with any medicine, taking it the way it’s designed to be taken is of utmost importance. If you don’t, you could end up becoming pregnant.

Knowing what can make your birth control ineffective and how you can increase your chances of successfully avoiding pregnancy will help you make the best decisions for you and your lifestyle.