A pregnancy test uses a blood or urine sample to determine whether or not a woman is pregnant. Both types of tests look for the presence of the hormone called human chorionic gonadotropin (HCG). When this hormone is detected, it almost always means the woman is pregnant.

Urine tests can be taken by a woman on her own. They are inexpensive and can be purchased at a pharmacy and other stores without a prescription. In addition, results from the test come back in as little as two minutes. They are usually accurate, with most manufacturers claiming an accuracy rate of at least 97 percent. Positive results of a home pregnancy test should always be confirmed by a physician.

Blood tests are taken in a physician’s office, clinic or laboratory. They are more expensive than urine tests, and results take longer to be returned. However, blood tests are even more sensitive and accurate than urine tests, and are useful in monitoring and detecting problems during pregnancy.

There are no significant risks associated with urine pregnancy tests. However, blood pregnancy tests include the minor risks associated with any blood test such as excessive bleeding, fainting and infection.

Pregnancy tests are used to determine whether or not a woman is pregnant. Some can be performed by a woman at home, and others must be administered in a physician’s office, clinic or laboratory. These tests look for the presence of the hormone human chorionic gonadotropin (HCG) in urine or blood.

As soon as a woman becomes pregnant, the developing embryonic and placental tissue begins releasing HCG into the bloodstream. This placental tissue is the vascular structure that attaches the fetus to the uterus. It provides oxygen and nutrients to and transfers wastes from the fetus. The placenta produces HCG shortly after the embryo attaches to the uterine lining, and the hormone remains present in the blood throughout pregnancy. During early pregnancy, the amount of HCG present in the blood doubles every two to three days. Its presence in a blood test or urine test almost always means the woman is pregnant. The measurement of the actual levels and change in levels helps determine the viability and health of the ongoing pregnancy.

Home pregnancy tests are reasonably inexpensive and can be purchased from a pharmacy and other stores without a prescription. Most manufacturers claim an effectiveness rate of 97 percent or higher. Positive results of a home pregnancy test should always be confirmed by a blood test. Blood tests are more expensive than urine tests and must be performed in a physician’s office. However, they are even more accurate than urine tests and they provide the actual levels of the hormone.

Although pregnancy tests are commonly used to confirm suspected pregnancies, physicians may also order the tests before performing a medical procedure or prescribing a drug that may negatively affect pregnancy.

When a pregnancy is confirmed, additional tests are offered later to assess fetal health. For example, a blood test for alpha-fetoprotein (AFP), often performed between the 15^th and 18^th week of pregnancy, can indicate chromosomal disorders such as Down syndrome or neural tube defects such as spina bifida.

Home pregnancy tests search for the presence of human chorionic gonadotropin (HCG) in the urine. They can detect HCG levels above 25 to 50 milli-international units per milliliter (mIU/mL), and can usually detect pregnancy about two weeks after conception. For this reason, a woman should not take a home pregnancy test until seven to 10 days after she has missed her period. However, some tests are available that claim to reveal pregnancy up to five days earlier than conventional tests.

There are three types of home pregnancy tests. They include:

• Dipstick test. The woman places the end of the dipstick in her urine stream during urination or immerses the dipstick in a container of collected urine for five to 10 seconds. Within a couple of minutes, the strip will reveal whether or not the woman is pregnant. Different manufacturers have their own methods for revealing the result. Examples include a plus or minus sign, a color change and the words “pregnant” or “not pregnant.”  Some women find it difficult to discern whether the color on the dipstick matches the test-kit control color. In most kits, a positive result is indicated by a test line that is as dark as or darker than the control. Women should also be aware that test results often change after an hour or two and are no longer accurate.
  
  
• Collection cup test. A sample of urine is collected midstream in a cup just after the woman has begun to urinate. The sample is then aspirated into an eyedropper, and several drops are placed onto a test disc provided with the pregnancy kit. Pregnancy is indicated by one color, and a negative result is indicated by another color.
  
  
• Chemical mixing test. A small amount of urine is placed in a test tube with a liquid or powder chemical mixture. If the chemicals make the urine change color, pregnancy is indicated.

There are few preparatory steps necessary before taking a home pregnancy test. Women will need to store, handle and use their test kits according to the manufacturer’s directions to avoid false results. They should check the test’s expiration date and refrain from drinking a lot of liquid beforehand because diluted urine may give a false negative result (pregnant but not indicated by test). It is best to take a home pregnancy test first thing in the morning, when urine is concentrated.

Most medications – whether over-the-counter or prescription – will not interfere with the test. This includes birth control pills (in instances where the woman may have gotten pregnant despite taking these pills) and antibiotics. However, some drugs may interfere with the results of a home pregnancy test. False positives (indicating pregnancy when there is none) may be caused by:

• Fertility drugs that contain human chorionic gonadotropin (HCG)
  
  
• Anticonvulsants (seizure medication)
  
  
• Anti-parkinsonism drugs
  
  
• Phenothiazines (used to treat mental and emotional disorders)
  
  
• Tranquilizers

Other factors can affect the accuracy of home pregnancy tests as well. Additional sources of false positives may include:

• Unclean collection cup. Soap or detergent residue can skew results.
  
  
• Old or damaged kit. Expired kits and those exposed to heat, dampness or vibration may be unreliable.
  
  
• Impure urine sample. Blood in the urine (such as from a urinary tract infection), protein (such as from kidney damage) and rare tumors that secrete HCG can give false results.
  

False negatives (indicating no pregnancy when it does exist) may result from:

• Diuretics (high blood pressure drugs)
• Promethazine (allergy medication)

Additional sources of false negatives may include:

• Taking the test too early. Women should not take a home pregnancy test until their period is at least one day late, the earliest that any conventional home pregnancy test can detect pregnancy. In fact, most conventional tests are considered more reliable after a woman’s period is at least a week late. However, some tests are available that claim to reveal pregnancy up to five days earlier than conventional tests.
  
  
• Using improper test timing. The test should be performed within 15 minutes of collecting the urine sample. Once beginning the test, the directions must be closely followed and timed according to instructions.
  
  
• Improper collection. Adequate amount and proper placement of the urine is necessary to determine if a woman is pregnant. If the dipstick is not in the urine stream, the test may not obtain enough urine to measure the hormone HCG. Also, if the test stick comes in contact with water, it may dilute the urine sample.

Generally, once a woman has tested her urine sample, the result will appear within one or two minutes. Women who get a positive result after taking a home pregnancy test should immediately schedule a visit with their physician. Women who get a negative result from a home pregnancy test should nonetheless contact their physician if they continue to have symptoms of pregnancy, such as a missed period, nausea, breast tenderness and fatigue.

Manufacturers of home pregnancy tests usually contend that the tests are 97 to 99 percent accurate. However, the accuracy of the tests is dependent upon the user closely following directions. When in doubt about how to perform these tests, a woman can call the toll-free telephone number listed on the package.

Pregnancy blood tests, which search for the presence of human chorionic gonadotropin (HCG) in the blood, are more reliable than home pregnancy tests, which detect HCG in the urine. Blood tests can detect pregnancy as early as six to eight days after conception. They can also measure the quantity of HCG in the blood rather than just its presence as with urine tests. Blood tests can also provide important information about the progress of a pregnancy. Blood tests are often used to confirm the results of home pregnancy urine tests. There are two types of pregnancy blood tests.

• Qualitative blood test. Detects the presence of HCG to indicate whether or not a woman is pregnant. Qualitative tests can detect HCG levels above 5 to 10 milli-international units per milliliter (mIU/mL).
  
  
• Quantitative blood test. This test measures the amount of HCG in the blood, ranging from 5 mIU/mL to 2 million mIU/mL or more. A quantitative blood test is typically performed when a yes or no answer is insufficient. It is useful in monitoring and detecting problems during the pregnancy.
  
  HCG levels normally rise throughout the first trimester and subsequently fall. Levels that fail to rise during pregnancy may indicate a number of complications (e.g., miscarriage, ectopic pregnancy, failing pregnancy). On the other hand, extremely high levels of HCG may suggest a multiple pregnancy. Higher-than-normal levels may also indicate cancer (e.g., ovarian cancer) or a rare type of placental disorder called molar pregnancy.

A pregnancy blood test may be performed in a physician’s office, clinic or laboratory. Prior to the test, the puncture site is cleaned with an antiseptic and an elastic band is wrapped around the patient’s upper arm. This applies pressure and restricts blood flow through the vein, causing blood to pool in the veins below the band.

A needle is then inserted into the vein (venipuncture), usually on the inside of the elbow or the back of the hand, and blood is collected into an airtight vial or syringe. Some patients may feel a moderate level of pain when the needle is inserted, while others feel only a prick or stinging sensation. Any discomfort associated with the blood drawing process typically lasts only a few seconds. The band is removed to restore circulation. Then the needle is removed and the puncture site is covered to stop any bleeding. After the sample is collected, patients may experience minor throbbing at the insertion site.

The sample is then sent to a laboratory for analysis. Results may be available anywhere from a few hours to a day after the blood has been drawn.

False positive blood test results may occur when certain hormones are elevated in the bloodstream. This is most likely when a woman is taking hormone supplements after menopause.

False negative results can result from several factors, including:

• Blood in the urine
• Protein in the urine
• Use of tranquilizer medications
• Use of penicillin or methadone
• Urinary tract infections
• Hepatitis

There are no significant risks associated with urine pregnancy tests. Blood pregnancy tests include the minor risks associated with any blood test. These include:

• Excessive bleeding
• Fainting or feeling lightheaded
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their condition. Patients may wish to ask their doctor or healthcare professional the following questions about pregnancy tests:

1. How accurate are urine pregnancy tests?
   
   
2. How soon after a missed period can I take a pregnancy test?
   
   
3. Where can I go for a blood test for pregnancy?
   
   
4. Can my (human chorionic gonadotropin) HCG levels be elevated by something other than pregnancy?
   
   
5. Does a negative result mean I cannot be pregnant?
   
   
6. Can a blood test tell how far along my pregnancy is?
   
   
7. What other tests might be needed?
   
   
8. What is HCG?
   
   
9. What does it mean if a test shows unusual levels of this hormone?
   
   
10. Can any of my current medications affect the accuracy of a pregnancy test?
    
    
11. What should I do if the test indicates no pregnancy but I still have symptoms?