How do at-home pregnancy tests work?

At-home pregnancy tests are designed to detect one hormone in particular: hCG. Although some use an ovulation test as a pregnancy test, it is still better to use a pregnancy test that detects this hormone.

hCG, or human chorionic gonadotropin, is a chemical that crops up when a new baby begins growing inside of you. It’s emitted by the tissues found in a human embryo—the earliest stage of a baby’s growth.

When a fertilized egg implants in your uterine wall, the placenta that will later encase your baby begins to form, triggering the release of hCG hormone. Over the first eight weeks of pregnancy, hCG hormone levels don’t just hit the gas pedal—they accelerate, nearly doubling every day.

In the earliest weeks after conception, hCG’s main role is to tell your body, “Hey, we’re pregnant!” Over the course of your first trimester, especially between 8 and 11 weeks, hCG assists with:

• Pausing menstruation
• Thickening the uterine lining
• Creating your placenta
• Promoting fetal growth
• Maintaining estrogen and progesterone levels

What are the different types of pregnancy tests?

Pregnancy tests come in various forms, each detecting the pregnancy hormone hCG in different ways. Here are the key differences between them:

• Urine pregnancy tests — These are the most commonly used tests, typically found in home pregnancy test kits or in a doctor’s office. They detect hCG in your urine, and are highly accurate when taken after a missed period.
• Blood pregnancy tests — These tests are usually performed in a lab and are used for more specific circumstances. A blood sample is taken from your arm to detect both the presence and quantity of hCG, providing a more precise reading.
• Saliva pregnancy tests — These tests detect hCG in your saliva, but their accuracy is lower than urine or blood tests. Saliva tests typically require you to wait until after your missed period, as the levels of hCG are lower in saliva.

When can I take a pregnancy test?

So, when should you take a test, and how soon will a pregnancy test read positive?

While plenty of at-home tests flaunt “99% Accuracy!” on their packaging, the truth is that this statistic depends on when they’re used. Small amounts of hCG can appear in your body as early as 11 days into a pregnancy. However, for most women, it takes at least two weeks after conception for hCG levels to be high enough for an at-home test to detect.

Most pregnancy tests require your urine’s hCG levels to meet a threshold of 20 to 50 milliliters in your urine stream. The earlier you test, the less likely you are to hit this benchmark.

Because of this, pregnancy test accuracy rates vary considerably in the days leading up to your first missed period:

• 6 days before: 25% accurate result
• 5 days before: 33% accurate result
• 4 days before: 42% accurate result
• 3 days before: 68% accurate result
• 2 days before: 81% accurate result
• 1 day before: 93% accurate result
• Expected day: 96% accurate result
• 1 day after: 99% accurate or higher

Ultimately, knowing when to take a pregnancy test comes down to tracking your menstrual cycle. But if you’re among the 25% of women who have irregular periods, you may yield an inaccurate result—even if you’ve waited those recommended 3 weeks. If this applies, look into when to test for pregnancy with irregular periods to avoid a false reading.

How accurate are pregnancy tests?

For a $20 pregnancy testing kit picked from your local drugstore, most at-home pregnancy tests are remarkably accurate when used as advised.

That said, the FDA estimates that up to 20% of pregnant women get false negative results (meaning, the test indicates you aren’t pregnant when you actually are) when testing on the assumed date of their first missed period.

A false negative result may be caused by:

• Taking the test too early (i.e. accidentally taking the test before the first day of your missed period)
• Misusing the test or checking your results too early
• Using watered-down urine with your test

Getting a false positive result from a pregnancy test is even less common, occurring at a rate of less than 1%. However, it’s not impossible. Usually, a false positive result is caused by:

• Excess protein or blood in your urine
• Certain medications (e.g. aspirin)
• An ectopic pregnancy (implantation that occurs outside the uterus, most commonly in the fallopian tube)

Note that while an ectopic pregnancy is technically a pregnancy, ectopic pregnancies are not viable. They must be medically terminated, otherwise, the mother could risk hemorrhaging and even death.

If you take an at-home test and receive positive test results alongside pelvic pain or bleeding, this may be an indication of an ectopic pregnancy. If you notice ectopic pregnancy symptoms, contact your OBGYN or healthcare provider right away.

4 signs you should take a pregnancy test

Learning to read your body’s cues can be enormously helpful for indicating a brand-new pregnancy.

Here are four reliable first signs of pregnancy that indicate it’s time to take a test:

1. Late or missed period – The first pregnancy symptom you might notice easily is a missed or late period. All healthy pregnancies will pause menstruation, so if you’re sexually active and your period misses its monthly appointment, that’s usually the first sign that your beautiful bun is in the oven. The only exceptions to the missed period rule apply to women with irregular cycles or amenorrhea (the complete absence of menstruation), or women who’ve reached menopause.
2. Sore breasts – Thanks to those tell-tale hormonal shifts, many women experience sore breasts in early pregnancy. At times, breast swelling and soreness can occur even before tests can accurately detect a bun in the oven!
3. Nausea – Ah, the dreaded “morning sickness.” While not clinically proven, research suggests that spiking hCG levels may trigger nausea and vomiting early in the first trimester. Some 70% of pregnant women experience morning sickness, so if you’re feeling queasier than usual, it may be time to take a test.
4. Increased urination – Pregnancy increases your body’s blood supply, which means your kidneys must work overtime to filter your blood and excrete excess waste through your urine. If you’ve been making more visits to the bathroom lately, it could be a symptom of early pregnancy.

Tips for getting accurate pregnancy test results

If you get the timing of your pregnancy test right, the most difficult part is over, but there are some tricks of the trade that can improve the accuracy of your test results.

Once your test date rolls around, keep these testing tips top of mind:

• Test in the AM – The best time of day to take a pregnancy test is right after you wake up in the morning when your hCG levels are at their highest.
• Hydrate (but don’t go crazy) – You need hydration to provide a urine sample, but don’t go overboard! Chugging water will leave you with diluted urine, which can lower its hCG levels and increase your chances of getting a false negative.
• Unbox right before use – Much like the avocado in your fridge, pregnancy tests can also “go bad”. Unused testing stick accuracy declines when left out in the open, so be sure to use yours right after you unwrap.
• Inventory your medications – Some drugs, particularly hormonal medications like Clomid, Menopur, and Pergonal, can compromise the accuracy of pregnancy tests. For an extra dose of confidence, check with your healthcare provider to see if any medications you use may affect test results.
• Repeat your test – No matter what answer you receive, it’s always best to double check your test results. Even though tests claim 99% accuracy after your first missed period, studies show that up to 5% of negative results are inaccurate—even when the test is used correctly. To confirm your results, experts recommend testing again one week after your first test.

FAQs about pregnancy tests

Should you take a pregnancy test if you’ve bled?

Many women who suspect they’re pregnant notice spotting in their underwear and forgo a test altogether—but this isn’t necessarily a sign that their period has come this month.

Around 25% of women experience implantation bleeding in early pregnancy after a fertilized egg embeds into the lining of their uterus.

While this can be a little scary for hopeful parents, it’s a completely normal process—albeit an inconvenient one. That’s because implantation bleeding usually lines up with your menstrual week, precisely the ideal time to take that test.

Because of this, it’s important to know how to spot (pun intended) the qualities that differentiate implantation bleeding from your usual period. Implantation bleeding tends to be:

• Light in flow
• Faintly pink- or brown-colored
• Free of tissue or clotted blood
• Mixed with cervical mucus

So, how soon after implantation bleeding can you test for pregnancy? Implantation typically only lasts for 1 to 2 days, so if your spotting recedes rapidly, it may be a sign to take a test.

What should I do if my pregnancy test is positive?

If your pregnancy test is positive, it’s important to schedule a visit to your healthcare provider for confirmation. They may perform a blood test or ultrasound to ensure the pregnancy is progressing normally. Begin prenatal care by taking care of your health, including eating a balanced diet, avoiding harmful substances, and starting prenatal vitamins with folic acid. It’s also a good time to discuss your options and prepare for the next steps in your pregnancy journey.

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