Birth control pills are designed to prevent pregnancy in a few key ways.

First, the pill stops monthly ovulation. Ovulation is the release of a mature egg. If that egg meets a sperm, pregnancy can occur.

Second, birth control pills make the lining of the cervix difficult for sperm to penetrate. Specifically, the cervix develops thick, sticky mucus. Sperm have great difficulty getting past this mucus, which reduces your chances of becoming pregnant.

If taken correctly, birth control pills are up to 99 percent effective in preventing pregnancy.

That’s an exceptionally high success rate, but it isn’t 100 percent. You could still get pregnant. For that reason, you may want to take a pregnancy test from time to time if you’re sexually active and think you might be pregnant.

You may wonder if the hormones in your birth control pills will affect a test’s outcome. Read on to discover some things to keep in mind if you’re on the pill and taking a pregnancy test.

The hormones in your birth control pills won’t affect a pregnancy test’s outcome.

However, some birth control pills affect the lining of your uterus. The hormones in birth control pills thin the lining. This makes it difficult for a fertilized egg to attach.

Without that lining, you may also not have a period or any bleeding. This may be mistaken for a pregnancy. That is just one of the reasons why you could suspect you’re pregnant even though you’re taking the pill properly.

“Perfect use” requires you to take the pill every single day at the same time without skipping a dose or being late to start a new pill pack.

When taken perfectly, birth control pills are 99 percent effective in preventing pregnancy. However, most people don’t take birth control pills in this manner.

“Typical use” refers to the way most people take the pill. That may mean they’re several hours late to take their dose or they miss a dose or two in any given month. In this case, the pill is only 91 percent effective in preventing pregnancy.

Aiming for perfect use can help increase the effectiveness of this birth control method. Once you’re in the habit of taking your pill at the same time every day, it’s important to maintain this routine.

You can do this by taking one pill a day until you’ve taken all of the pills in your pack, including the placebo pills.

Placebo pills have little to no active ingredients but help you keep the schedule of taking a daily pill. Keeping your daily routine going can ensure you don’t accidentally forget to start your next pack.

If you do skip or miss a dose, play it safe and use backup protection, such as a condom, for at least a week. If you went more than a day or two without a dose, it might be safer to use a backup method for up to a month.

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Set a pill reminder The birth control pill is designed to keep hormone levels in your body even. If you skip a dose or are several hours late, your hormone levels can drop, which may trigger ovulation. Set a reminder on your phone so you can take your pill every day at the same time.

The earliest symptoms of pregnancy can be easy to miss. If you notice any of the symptoms below, take a pregnancy test to find out your status.

Morning sickness

Morning sickness may be one of the first signs of pregnancy. Although it’s most common in the morning, it can occur any time of day. Morning sickness involves nausea or vomiting. It can begin within a few weeks of conception.

Breast changes

Early pregnancy hormonal changes may leave your breasts feeling tender and sore. They may also swell or feel heavier.

Missed period

A missed period is often the first sign of pregnancy in many cases. If you’re on birth control, you may not get regular periods, so a missed period may be hard to determine.

Fatigue

Changes to your body in early pregnancy may leave you feeling tired and sluggish more easily.

Frequent urination

Urinating more than usual may be a symptom of pregnancy.

Changes in eating patterns

Suddenly developing food aversions may be a symptom of early pregnancy. Sense of smell is heightened in early pregnancy, and your taste for some foods might change. Food cravings can develop, too.

The hormones in birth control pills can also change your eating patterns, so it may be hard to determine what’s causing your sudden palate shift.

Over-the-counter (OTC) pregnancy tests detect the level of a hormone called human chorionic gonadotropin (hCG). Pregnancy tests can detect this hormone 99 percent of the time if used correctly.

Here’s how to ensure you get the most accurate result possible:

1. Pay close attention to the test’s instructions

Each test is different, so before you open the package, be sure to read the instructions. Keep a timer handy if you need to time your test.

2. Wait for the right time to take the test

Your hCG levels will begin to climb once the fertilized egg is implanted. For some, this may not be until the first day of your period. If you can wait until after your missed period, tests may be more accurate.

3. Take the test in the morning

Your levels of hCG will be highest after you wake up because you haven’t urinated yet.

4. Research the tests you get

Some pregnancy tests tout that they can detect a pregnancy days before you miss a period. These tests are more sensitive than more traditional tests. Which test you use may affect how soon you can know if you’re pregnant.

Buy now: Shop for pregnancy tests.

Though pregnancy tests are highly accurate, there’s still room for error. A few issues can affect your results, but your birth control pill isn’t one of them. The hormones in your birth control pill don’t affect a test’s ability to detect hCG.

Some possible issues are described below. There are other, less common reasons not listed here.

Reading the test incorrectly

Differentiating between two faint blue lines and only one may be difficult. This is especially true if your levels of hCG are very low and the test isn’t very sensitive to the hormone.

Wait a few days and test again if you think your result was difficult to read.

Using the test incorrectly

Each test comes with very specific instructions. It’s possible for you to make an error during testing.

For example, some tests give results in as few as two minutes, but the results aren’t valid after 10 minutes. This is because the results might change due to the test’s design. Other tests require you to wait at least 10 minutes for a result.

Not knowing how your test functions could lead to an inaccurate result.

Using an expired test

Don’t risk a false test result by using an expired test. Once the “use by” date has passed, pitch the sticks and buy new ones.

Taking the test too soon

Your levels of hCG will increase quickly once a fertilized egg is in place. If you take your test too soon, the hormone levels may not be high enough yet for a test to detect. It’s recommended you wait until you’ve missed your period to take the test.

Picking the wrong test for your needs

If you want to test for a possible pregnancy before your missed period, pick a test that’s designed to test that early. The test will have to be very sensitive to get an accurate result.

If you use a more traditional test before your missed period, the test may not be able to detect the hormone.

While at-home urine pregnancy tests are very accurate, they aren’t 100 percent accurate. Blood tests done by your doctor are 100 percent accurate, however. If you want further confirmation of your pregnancy status, make an appointment with your doctor.

They’ll draw a quick blood sample and send it for testing. In some cases, you can know within a matter of minutes whether you’re pregnant or not. Otherwise, you may have to wait two to three days for your results to return.

If you’re unsure whether you should take a pregnancy test, always err on the side of caution. Take one if it’ll help reduce your anxiety. You also can and should take pregnancy tests while you’re using birth control if you want to know your pregnancy status.

Consider asking your doctor about signs and symptoms that may indicate the need for a pregnancy test. Some of the earliest symptoms of pregnancy can go undetected. Your doctor may give you more specific symptoms to look for before you take a test.

If you become pregnant, it’s good to know as soon as possible. Knowing early allows you to better prepare for what comes next.