To confirm a pregnancy; to help identify an ectopic pregnancy; to monitor a pregnancy that may be at risk of failing; occasionally to screen a woman for pregnancy prior to some medical treatments; as part of a panel of tests used to screen for fetal abnormalities (see First Trimester Down Syndrome Screen and Second Trimester Maternal Serum Screening)
Pregnancy Test (hCG)
When you suspect that you are pregnant (testing can be done reliably by 10 days after you miss your menstrual period and some methods can detect hCG even earlier); when you have signs and symptoms that suggest you may have an ectopic pregnancy or a pregnancy that is failing; prior to some medical treatments
A urine sample, preferably collected first thing in the morning, or a blood sample drawn from a vein in your arm
Do not drink large amounts of fluid before collecting a urine sample for a pregnancy test because overly diluted urine may result in a false negative; no preparation is needed for a blood sample.
Human chorionic gonadotropin (hCG) is a hormone produced by the placenta of a pregnant woman. Early in pregnancy, the level of hCG increases in the blood and is eliminated in the urine. A pregnancy test detects hCG in the blood or urine and confirms or rules out pregnancy.
During the early weeks of pregnancy, hCG is important in maintaining function of the corpus luteum. Production of hCG increases steadily during the first trimester (8-10 weeks) of a normal pregnancy, peaking around the 10th week after the last menstrual cycle. Levels then fall slowly during the remainder of the pregnancy. hCG is no longer detectable within a few weeks after delivery.
When a pregnancy occurs outside of the uterus (ectopic), the level of hCG in the blood increases at a slower rate. When an ectopic pregnancy is suspected, measuring the level of hCG in the blood (quantitative test) over time may be useful in helping to make a diagnosis of ectopic pregnancy.
Similarly, the hCG blood level may be abnormal when the developing baby (fetus) has a chromosome defect such as Down syndrome. An hCG test is used routinely in conjunction with a few other tests as part of screening for fetal abnormalities between 11 and 20 weeks of pregnancy. (See First Trimester Down Syndrome Screen or Second Trimester Maternal Serum Screening for more on this.)
- How is the test used?
Pregnancy hCG tests that give a positive or negative result (qualitative) detect the presence of hCG. These tests may be performed by a laboratory, at a healthcare practitioner's office, or at home using a home pregnancy test kit. Methods will vary slightly but for most, a test strip is dipped into a collected cup of urine or exposed to a woman's urine stream. A colored line (or other color change) appears within the time allotted per instructions, usually about 5 minutes. For accurate test results, it is important to carefully follow the test directions. (See the article on Home Testing for more on this.) If the test is negative, it is often repeated several days later. Since hCG rises rapidly, an initial negative test can turn positive within this time period.
Quantitative hCG testing, often called beta hCG (β-hCG), measures the amount of hCG present in the blood. It may be used to:
- Confirm a pregnancy
- Help diagnose an ectopic pregnancy, along with a progesterone test
- Help diagnose and monitor a pregnancy that may be failing
- Monitor a woman after a miscarriage
hCG blood measurements may also be used, along with a few other tests, as part of screening for fetal abnormalities. For more information on this use, see First Trimester Down Syndrome Screen or Second Trimester Maternal Serum Screening.
A quantitative hCG blood test may also be used as a tumor marker in some cancers. For more on this, see the article hCG Tumor Marker.
Occasionally, an hCG test is used to screen for pregnancy if a woman is to undergo a medical treatment, be placed on certain drugs, or have other testing, such as x-rays, that might harm the developing baby. This is usually done to help confirm that the woman is not pregnant. It has become standard practice at most institutions to screen all female patients for pregnancy using a urine or blood hCG test before a medical intervention, such as an operation, that could potentially harm a fetus.
- When is it ordered?
For confirming pregnancy, the timing of testing depends on how accurate a woman is about the day she expects her menstrual period as well as the method used for testing. In general, blood tests are more sensitive than urine tests and can be done two days before a woman would expect her period to start. A urine or blood hCG test can be done reliably by 10 days after a missed menstrual period. Even using a urine test, a woman may be able to determine whether she is pregnant the day she misses her period, but the result could be falsely negative. Testing may be repeated at a later date if the first test is negative but pregnancy is still suspected.
Quantitative blood hCG tests may be ordered repeatedly over several days when a healthcare practitioner wants to identify or rule out an ectopic pregnancy or to monitor a woman after a miscarriage. In these cases, a woman may experience the normal signs and symptoms of pregnancy at first but then may develop others that indicate that the pregnancy is not progressing as expected.
Some signs and symptoms of ectopic pregnancy include:
- Abnormal vaginal bleeding—because a woman is pregnant, she may not have a regular period but then may have light bleeding or spotting with an ectopic pregnancy
- Low back pain
- Pain or cramping in the lower abdomen or on one side of the pelvis
If untreated, signs and symptoms may get worse and may include:
- Dizziness, weakness
- Feeling faint or fainting
- Low blood pressure
- Pain in the shoulder area
- Sudden, sharp pain in the pelvic area
- Fever, flu-like symptoms
The area around an ectopic pregnancy may rupture and start to bleed, and, if undiagnosed, can lead to cardiac arrest and death.
An hCG test may be ordered prior to a medical procedure or treatment that might be harmful during pregnancy.
- What does the test result mean?
A negative hCG result means that it is unlikely that a woman is pregnant. However, tests performed too early in a pregnancy, before there is a significant hCG level, may give false-negative results. The test may be repeated in one week if there is a strong possibility of pregnancy.
A positive hCG means that a woman is likely pregnant. However, false-positive tests can occur if a woman is post-menopausal or taking hormone supplements.
The blood level of hcG in a woman with an ectopic pregnancy usually rises at a slower rate than normal. Typically, hCG levels double every 48 to 72 hours for the first four weeks of a normal pregnancy, then slow to every 96 hours by six weeks. Those with failing pregnancies will also frequently have a longer doubling time early on or may even show falling hCG concentrations during the doubling period. hCG concentrations will drop rapidly following a miscarriage. If hCG does not fall to undetectable levels, it may indicate remaining hCG-producing tissue that will need to be removed (dilation and curettage D&C).
A high hCG level may indicate that the pregnancy dating has been miscalculated, that there is a molar pregnancy, or that it is a multiple pregnancy.
- What can cause a false-negative urine hCG?
Urine hCG tests may give a false-negative result if the urine is too diluted or if testing is done too soon in the pregnancy. Certain drugs such as diuretics and promethazine (an antihistamine) may cause false-negative urine results.
- What can cause a false-positive result?
Drugs such as antihistamines, anti-anxiety medications, diuretics, anti-convulsants, anti-parkinson drugs, hypnotics, and tranquilizers may cause false-positive results.
The presence of protein in the urine (proteinuria), blood in the urine (hematuria), or excess pituitary gonadotropin may also cause a false-positive urine hCG test.
There are reports of false-positive blood hCG results due to the presence of certain types of antibodies that some individuals produce or fragments of the hCG molecule. Generally, if results are questionable, they may be confirmed by testing with a different method.
- How does the test that I do at home myself compare with the results of a test done in a lab?
Home pregnancy testing is very similar to qualitative urine hCG testing performed in the laboratory, but there are factors surrounding its use that are important to note.
- Home tests come with very specific directions that must be followed explicitly. If you are using a home test, follow the directions extremely carefully (see Home Testing). There can be variability in sensitivity to detecting the presence of hCG with different brands of home pregnancy kits.
- Home tests are sometimes done too soon after the missed menstrual cycle to result in a positive test. It typically takes 10 days after a missed menstrual period before the presence of hCG can be detected by the urine test.
- All urine hCG tests should be done on a first morning urine sample, if possible. Urine becomes more dilute after ingestion of liquids (coffee, juice, water, etc.) and urine hCG concentrations may become too low to register as positive.
Generally, when used correctly, the home test should produce the same result as the urine hCG test done by your healthcare practitioner. Blood testing for hCG is more sensitive than urine hCG testing, so sometimes a blood test will indicate pregnancy when the urine test is negative.
- When is a blood hCG test ordered instead of a urine hCG?
Since hCG is not normally detected in the urine of a non-pregnant woman, a urine hCG is enough to confirm a pregnancy. This can also be done with a qualitative blood hCG test. Sometimes, however, it is important to know how much hCG is present to evaluate a suspected ectopic pregnancy or to monitor a woman following a miscarriage. In these circumstances, a healthcare practitioner will order a quantitative blood hCG test.
- How many days after a miscarriage would it take for a urine pregnancy test to show a negative result?
Urine hCG decreases at about the same rate as serum hCG, which can take anywhere from 9 to 35 days, with a median of 19 days. However, the timeframe for when an hCG result will be negative is dependent on what the hCG level was at the time of the miscarriage. Frequently, miscarriages are monitored with quantitative blood hCG testing. If the levels of hCG do not fall to undetectable levels, some hCG-producing tissue may remain and have to be removed.
- What is an ectopic pregnancy?
An ectopic pregnancy occurs when the fertilized egg (ovum) implants somewhere other than in the uterus. Nearly all ectopic pregnancies occur in the fallopian tube, giving the condition the moniker of "tubal pregnancy." This is a serious condition needing immediate treatment. Women with ectopic pregnancies often have sharp, stabbing abdominal or pelvic pain and uterine bleeding. Usually, abnormally low levels of hCG are produced in ectopic pregnancies with slower-than-normal rates of increase. A physician that suspects an ectopic pregnancy might also test progesterone levels, which will be lower than in a non-ectopic pregnancy.
In the case of an ectopic pregnancy, hCG levels need to be rechecked regularly until they are undetectable. If hCG levels remain high, it might mean that the ectopic tissue wasn't removed, which could require treatment by surgery or methotrexate, which causes the body to absorb the pregnancy tissue.