A pregnancy test can let an individual know if a woman is pregnant. Although they are not quite 100 percent accurate, they are incredibly reliable and widely used.
This article will discuss how pregnancy tests work, when they can be used, and how accurate they are.
Contents of this article:
Fast facts on pregnancy tests
Here are some key points about pregnancy tests. More detail and supporting information is in the main article.
- Pregnancy tests work by detecting human chorionic gonadotropin
- Certain rare conditions can produce false readings in pregnancy tests
- Each brand of pregnancy works slightly differently, so it is important to read the instructions before use
- If completed properly, pregnancy tests can be 97 percent accurate
- Some medications can alter pregnancy test readings
What is a pregnancy test?
Some sensitive pregnancy tests can be completed as early as 4 days before a period is due.
A pregnancy test works by measuring the amount of a particular hormone which increases during pregnancy. This hormone is called human chorionic gonadotropin (HCG).
HCG can be present in the blood and urine approximately 10-14 days following conception and peaks between 8 and 11 weeks gestation.
HCG is known as the pregnancy hormone because it is produced by the cells that form the placenta and provides nourishment to the growing embryo.
A negative HCG result is a level less than 5 mIU/ml (milli-international units per milliliter), while a positive HCG for pregnancy is greater than or equal to 25 mIU/ml.
As a side note, pregnancy tests can also be used to diagnose other conditions that raise HCG levels, including transitional cell carcinoma of the bladder and urinary tract, prostate cancer, renal cancer, and cancers of the gastrointestinal system.
How to perform a pregnancy test
Pregnancy tests can be performed in one of two ways: urine or blood testing.
When testing at home, the urine is placed on a chemical strip and produces a result in approximately 1-2 minutes, although different brands of test may vary in the result times.
These tests can be performed by collecting urine in a collection cup and either dipping the pregnancy test stick into the urine or placing the urine into a container using an eyedropper. Alternatively, a pregnancy stick can be placed into the urine stream.
Each brand of testing has its own method of giving results, so it is important to read the package insert to determine how the results are given.
Some tests change in color, others have line changes or provide symbols such as a plus and minus. Other tests, including some digital ones, may provide a simple answer in the test window, for example, the words "pregnant" or "not pregnant."
When checking for pregnancy using blood, there are two forms of test: a quantitative and qualitative HCG test.
A quantitative blood test is used to measure the exact amount of HCG present in the blood, while the qualitative test simply tells you if HCG is there or not.
There are some advantages to blood testing, for instance, early pregnancy detection and the ability to measure HCG concentration; however, there are also disadvantages such as high cost, a long wait for results, and the need to have the test completed in a medical office.
When to take a pregnancy test and what the results mean
Some tests have line changes, providing a symbol such as a plus or minus - the top test is negative, and the one below is positive.
When taking a home urine test, it is best to wait until a period is missed. However, 14 days from possible conception is the earliest possible time to take the test.
Some tests on the market can be taken earlier and depend on their individual sensitivity; it is important to read the package insert to determine the best and earliest time to take each particular test.
The best time of day to take a urine pregnancy test is first thing in the morning, after waking. If the woman has consumed a lot of liquids prior to taking the test, it can produce a false negative, which is part of the reason why early morning is the best time to get a reliable result.
A positive home pregnancy test simply means that HCG is present in the urine, while a negative test can mean a variety of things.
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.
Accuracy of pregnancy tests
When taken correctly, following the instructions carefully, urine pregnancy tests claim to offer approximately 97 percent accuracy. However, incorrect use of the test can result in a reading that is potentially inaccurate.
If at first you receive a negative result but are exhibiting symptoms of pregnancy, it is recommended that you re-test in 1 week or speak with 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, these are not as accurate as the pregnancy tests themselves.
There are certain medications which can affect a pregnancy test's accuracy; these include:
- Promethazine - used to treat certain allergies
- Parkinson's disease medication
- Sleeping tablets (hypnotics)
- Some antipsychotics, including chlorpromazine
- Opioids - for instance, methadone
- Anticonvulsants - for instance, epilepsy treatments
- Infertility medication
Alcohol in the blood stream does not affect the pregnancy test because it does not interfere with the hormone levels being measured. However, if an individual is trying to get pregnant or expecting to fall pregnant, alcohol use is not recommended.
Drinking during early pregnancy can increase the risk of miscarriage, premature birth, and low birth weight.
Certain medical conditions cause a rise in HCG, and can, therefore, give false positives on a pregnancy test; these are very rare, but include:
- Deficiencies of immunoglobulin A (IgA) - an antibody important in immune function
- Gestational trophoblastic disease - rare pregnancy-related tumors
- Gestational trophoblastic neoplasms - rare uterine tumors