A pregnancy test is around 99 percent reliable.
It works by measuring levels of a hormone called human chorionic gonadotropin (HCG). HCG can be present in the blood and urine approximately 10 to 14 days after conception.
If a woman misses a period or other signs suggest she may be pregnant, it is a good idea to take a test.
A woman should know as early as she can if she is pregnant, as she will probably need to monitor her health more carefully, and she may need to make some lifestyle changes.
Here are some key points about pregnancy tests. More detail is in the main article.
- Pregnancy tests work by detecting human chorionic gonadotropin (HCG).
- Some conditions and some medications can produce false or inaccurate readings in pregnancy tests.
- Each brand of pregnancy test works slightly differently, so it is important to read the instructions before use.
- Used properly, pregnancy tests can be 99 percent accurate.
How does a pregnancy test work?
A pregnancy test is 99 percent accurate at letting a woman know if she is pregnant.
A pregnancy test works by measuring the amount of HCG hormone. HCG levels increase during pregnancy.
HCG is known as the pregnancy hormone, because it is produced by the cells that form the placenta and provide nourishment to the growing embryo.
It is released when a fertilized egg attaches itself to the lining of the uterus.
HCG can be present in the blood and urine around 10 to 14 days after conception. It peaks between 8 and 11 weeks of gestation.
- A negative HCG result is a level less than 5 mIU/ml (milli-international units per milliliter).
- A positive HCG for pregnancy is greater than or equal to 25 mIU/ml.
How do I take it?
Pregnancy tests involve testing urine or blood. A urine test can be self-administered at home, but a medical professional must perform a blood test.
Urine testing at home
A home test involves placing the urine on a chemical strip. The result is ready in around 1 to 2 minutes, although different brands of test may vary in the result times.
The individual collects urine in a container and either dips the pregnancy test stick into it or uses an eyedropper to place the sample on the test strip.
Alternatively, a pregnancy stick can be held in the urine stream.
Each brand of testing has its own way of giving results, so it is important to read the package insert to find out how to understand the results.
Depending on the test, the result may appear as:
- a change of color
- a change in a marked line, or the appearance of a symbol, such as a plus or minus
- an answer in a test window, for example, the words "pregnant" or "not pregnant"
A blood test can be a more accurate and detailed way to confirm pregnancy. However, a healthcare professional will need to administer this.
There are two ways to check for pregnancy using blood: a quantitative and qualitative HCG test.
- A quantitative blood test measures the exact amount of HCG present in the blood.
- A qualitative test detects whether HCG is present or not.
Blood testing has some advantages.
It allows for early pregnancy detection, and it can measure HCG concentration.
However, it is more expensive, the results can take time, and the test must be completed in a medical office.
When can I take it?
The earliest time for taking a home urine test is 14 days after possible conception, but waiting until a missed period will give a more reliable result.
Some tests on the market can be taken earlier. This depends on how sensitive they are.
It is important to read the package insert to know the best and earliest time to take the test.
The best time of day to take a urine pregnancy test is first thing in the morning, after waking.
This is because consuming a lot of liquids before taking the test, can lead to a false negative, even if the woman is pregnant.
What happens next?
Used correctly, a urine pregnancy test should be around 99 percent accurate. However, incorrect use can result in an inaccurate reading.
A positive home pregnancy test simply means that HCG is present in the urine, while a negative test can have a variety of meanings.
Negative tests can mean that there is no pregnancy, or they can mean that the test was either taken too early to detect HCG, or that it was not done correctly.
If the result is positive, the woman should make an appointment to see a healthcare professional, who can guide her about the next steps.
If the result is negative, but the woman has the signs and symptoms of pregnancy, she should test again after 1 week or ask a doctor about a blood test instead.
Some modern pregnancy tests and kits can give an estimate of the conception date and consequent due date. However, this information is not always accurate.
Can medications affect the result?
Some medications can affect a pregnancy test's accuracy.
- Promethazine, used to treat certain allergies
- Parkinson's disease medication
- Sleeping tablets
- Some antipsychotics, including chlorpromazine
- Opioids, for instance, methadone
- Anticonvulsants, including epilepsy treatments
- Infertility medication
Alcohol in the blood stream does not affect the pregnancy test because it does not interfere with the measurement of the hormone levels.
However, anyone who is trying to or expecting to become pregnant should avoid alcohol, as it can affect fetal development. Drinking during early pregnancy can increase the risk of miscarriage, premature birth, and low birth weight.
Other factors that affect HCG
Sometimes, there may be a positive test result but the woman is not pregnant. High HCG can also indicate that some health condition has increased levels of the hormone.
This could be:
- a deficiency of immunoglobulin A (IgA), an important antibody in immune function
- gestational trophoblastic disease, a rare, pregnancy-related tumor
- gestational trophoblastic neoplasms, a rare uterine tumor
HCG can rise as a result of some types of cancer. These are transitional cell carcinoma of the bladder and urinary tract, prostate cancer, renal cancer, and cancers of the gastrointestinal system. A pregnancy test may be used to assess for these.
Pregnancy tests are available at drugstores, pharmacies, and some convenience stores. Home pregnancy tests can be purchased for as little as a dollar, and some health centers offer them free of charge.