A home pregnancy test detects human chorionic gonadotropin (hCG), the pregnancy hormone. Although there are many different types and brands of home pregnancy tests, they all function in basically the same manner: They check the urine to detect the presence of hCG.﻿﻿

Stay Calm Mom: Episode 2 Watch all episodes of our Stay Calm Mom video series and follow along as our host Tiffany Small talks to a diverse group of women and top doctors to get real answers to the biggest pregnancy questions.

8:35 A Positive Pregnancy Test: Now What?

Types of Tests

Most home tests use a dipstick to collect and analyze urine. These one-step kits are usually considered the most convenient to use. You either hold the stick briefly in the urine stream or dip it into a cup of collected urine. You can also buy individual paper testing strips to dip into a cup of urine (these are very similar to the one-step kits, just without the plastic cover).

There are also home pregnancy tests that require a woman to mix a small amount of her urine with a special liquid or powder (though these are pretty rare). Even though each test works in a similar way by checking urine for the presence of hCG, it is still important to read the test’s instructions, as these can vary between brands. Performing or interpreting the test incorrectly could give you an inaccurate result.

How They Work

The female body only begins to release hCG when a fertilized egg implants in the uterus. In most women (but not all), this occurs about six days after conception. The hCG levels increase with each passing day of the pregnancy, doubling about every two days.

Home pregnancy tests can reliably detect this hormone approximately one week after a missed period. Although some home tests can detect hCG as early as the first day of a missed period, most are not sensitive enough to guarantee results if taken this soon.﻿﻿

Accuracy

Accuracy claims can be somewhat misleading. Many home pregnancy tests typically maintain a 99% accuracy rate or better. The problem lies in the fact that these tests also imply that this accuracy could be expected if you take the test as early as the day of a missed period, but that's not what they're testing for in the laboratory to get their 99% accuracy rate.

A 2004 study﻿﻿ published by Laurence Cole, MD, and researchers at the University of New Mexico confirmed the deceptive claims of many of these early pregnancy tests. The problem is that the amount of hCG occurring in the urine on any given day after implantation can vary significantly, Dr. Cole explains.

His research found that for early detection of pregnancy, most tests were not sensitive enough to detect hCG on the first or second day after a missed period.

Out of 18 brands tested, only one, the First Response Early Result Test, was sensitive enough to "consistently detect 12.5 mIU (milli-International Units per milliliter of urine) of hCG, considering the manufacturer's suggested read time, and produced both clear and faintly discernible positive results" on the first and second day after a missed period.

This level of sensitivity (12.5 mIU) is required to detect 95% of pregnancies at the time of a missed period.

Three of the brands tested were able to detect 80% of pregnancies on the first or second day of a missed period. Most of the others were only able to detect hCG in 16% of pregnancies when tested a day or two after a missed period. A more recent study (published in 2014﻿﻿ ) suggested that for many tests, "results do not match the claims made in the package insert."

Determining Test Efficacy

Usually, the more sensitive the test, the earlier you can get an accurate pregnancy test result. However, if a woman has hCG in her system from a recent birth or miscarriage, or from fertility drugs, a less-sensitive test may be a better option.

A test's package insert should explain the lowest mIU concentration of hCG that the test can detect. In theory, a pregnancy test that maintains that it could identify hCG at 25 mIU should be more sensitive than one that can identify this hormone at 40 mIU.

The only thing to be aware of is that a woman produces different kinds of hCG during pregnancy, so sometimes the sensitivity claims of pregnancy tests do not actually indicate that the test will pick up on the type of hCG most associated with early pregnancy.

Timing

The tests' 90% to 99% accuracy claims are generally true once a woman is further along in pregnancy—just not during the first few days, even though the package may say you can take the test on the first day of a missed period. This is why it is usually best to wait at least one week after a missed period to take a pregnancy test.

Test Results

Depending on the design of the test, the pregnancy test results may be simpler or harder to read. A test that has enough contrast between the line (or symbol) and the background makes the results easier to interpret. Some brands indicate that an evaporation line may appear if the test is left to sit past a certain time frame; this line may make it more difficult to accurately interpret the test results.

Negative Test Results

If you get a negative, or not pregnant, result on your pregnancy test, you might still be pregnant (this called a false negative, and it's more likely than a false positive). A false-negative test result can occur if:

Your urine is diluted. Many tests suggest that you perform the test in the morning, right after you wake up. This is because your urine is usually the most concentrated at this time. If you drink too much liquid before performing the test, you may end up with an inaccurate result.

Many tests suggest that you perform the test in the morning, right after you wake up. This is because your urine is usually the most concentrated at this time. If you drink too much liquid before performing the test, you may end up with an inaccurate result. You performed the test too soon. In order to at least have the possibility of an accurate result, you should wait at least a day after your missed period. However, to decrease the chances of a false-negative result, it is better to wait seven days after your period was due.

In order to at least have the possibility of an accurate result, you should wait at least a day after your missed period. However, to decrease the chances of a false-negative result, it is better to wait seven days after your period was due. You timed the test incorrectly. It is important that you perform the pregnancy test within 15 minutes after collecting a urine sample. Make sure that you follow the test’s instructions as to how long it takes to analyze the results. If you check the results too soon, the test result may appear to be negative. (If you check them too late, the result could be a false positive.)

If your period has not started within a week after a negative result, you should take another pregnancy test. If it's still negative, make an appointment with your health-care professional to determine what is going on. Circumstances like stress, excessive exercise, illness, and hormonal imbalances can also cause a woman to miss a period.

Positive Test Results

Typically, if you receive a positive result (even if it is very faint), this indicates that you are pregnant. While it is possible to obtain a false positive result (the test is positive, but you are really not pregnant), it happens very rarely. You may end up with a false positive if:

You are taking a prescription medication that contains hCG , such as Pregnyl, A.P.L., Profasi, Chorex, Novarel, or Ovidrel, or promethazine (used as an antihistamine in a combination cough and cold products to treat allergy symptoms and to treat nausea or vomiting from illness or motion sickness).

, such as Pregnyl, A.P.L., Profasi, Chorex, Novarel, or Ovidrel, or promethazine (used as an antihistamine in a combination cough and cold products to treat allergy symptoms and to treat nausea or vomiting from illness or motion sickness). There are traces of blood or protein in your urine.

in your urine. The pregnancy test you used was expired or damaged.

or damaged. You are currently taking diuretics.

You had a chemical pregnancy. This means that a fertilized egg did implant into your uterus and developed just enough to trigger the production of hCG, but then, for whatever reason, stopped developing. Typically, about 30% to 50% of all fertilized eggs end up as chemical pregnancies because of abnormalities or other reasons that make further development impossible. When this occurs, most women will end up getting a period (though it may be a few days later or heavier than usual).

The higher likelihood of a chemical pregnancy to occur is another reason why it may be better to wait at least a week after your period is due to take a home pregnancy test.

Where to Buy a Test

Most grocery stores, drug stores, and big-box stores sell home pregnancy tests over the counter (without needing a prescription). Depending on the brand and how many tests come in the box, tests can cost between $4 and $20.

If you think you may need a second test because you have irregular periods or you are testing right after a missed period, it is usually a better bargain to purchase a two-pack than paying separately for two tests.

Blood Tests vs. Home Tests

The urine pregnancy tests performed at most doctor’s offices are basically the same kind as the ones found over the counter. The main difference in pregnancy testing is that some healthcare professionals will use blood tests, which can detect pregnancy much earlier than urine tests can.

Another advantage of a blood test is that it can reveal the exact amount of hCG in the blood (not just whether or not it is present). This is helpful to assess how far into a pregnancy a woman may be or if there is the possibility that a woman may be miscarrying.

What to Do Next

If you receive a positive result on a home pregnancy test, make an appointment to see your healthcare provider. You should also see your doctor if you have taken a few home pregnancy tests and have received mixed results.

Your provider may perform a blood test or pelvic exam to confirm your result. The sooner you know whether or not you are pregnant, the sooner you can start to make decisions about your pregnancy.