What is being tested?
hCG is a protein hormone produced in the placenta of a pregnant woman. A pregnancy test is a specific blood or urine test that can detect hCG and confirm pregnancy. This hormone can be detected by a blood test as early as one week after conception or by a urine test at around the time of the first missed menstrual period.
hCG production doubles about every 48 hours for the first month of a pregnancy and continues to rise steadily, until about 10-12 weeks when levels drop until about the 20 week mark, and then continue largely the same until term.
It is important to note that there is a wide range of hCG levels between individuals, but this trend appears to be relatively constant. Estimates of gestational age are difficult to do using hCG due to this wide variation between individuals, so ultrasound investigations are used to check pregnancy duration. hCG is no longer detectable a few weeks after delivery.
During the early weeks of pregnancy, hCG is important in maintaining function of the corpus luteum (the mass of cells that forms from a mature egg into the developing embryo).
If something goes wrong with the pregnancy, the level of hCG may be higher or lower than usual. In the case of an ectopic pregnancy, the hCG may be lower than usual and, if a woman miscarries, the level of hCG will fall quickly. If a woman develops trophoblastic disease, the hCG level may be higher than expected.
HCG is also produced by some germ cell tumours.
Repeat testing will help to monitor the hCG levels where necessary in these situations.
How is it used?
Qualitative (reported as positive or negative - 'yes' or 'no') hCG testing is routinely used to confirm pregnancy. Urine tests, such as those done at home, are qualitative.
Quantitative (reported as a number) hCG testing measures the actual amount of hCG present. Blood tests give a quantitative result and may be used to help confirm that a woman is pregnant, but are particularly useful when something is going wrong with the pregnancy, such as in an ectopic pregnancy or in a threatened miscarriage.
In addition, a quantitative hCG test may be used to diagnose trophoblastic disease or germ cell tumours of the testes or ovary. It may also be used at regular intervals to monitor the effectiveness of treatment for these conditions and to detect tumour recurrence.
When is it requested?
To tell if a woman is pregnant, a blood test can be done that may detect hCG as early as 7 days after conception. A urine test may be able to detect hCG at around the time of the first missed menstrual period.
If something is going wrong with the pregnancy, the doctor may order hCG blood tests every so often to monitor the condition.
A doctor will also request an hCG blood test if he or she suspects trophoblastic disease or a germ cell tumour.
What does the result mean?
In non-pregnant women, hCG levels in the blood are normally very low or undetectable. During early pregnancy, the hCG level in the blood doubles about every two to three days. If something is going wrong with the pregnancy, the hCG does not rise as expected and may even fall. If a woman has a miscarriage, the hCG level in the blood falls rapidly. If it does not, it may indicate remaining hCG-producing tissue that will need to be removed.
hCG is also used to monitor treatment in patients with trophoblastic disease and to detect recurrent disease after treatment is complete. During therapy, a falling hCG level generally indicates that a tumour is responding to treatment, while rising levels may indicate that a tumour is not responding to therapy. Increased hCG levels after treatment may indicate a recurrence of disease.
Is there anything else I should know?
There are a number of forms of hCG and one type of hCG is also tested as part of the screening tests done on pregnant women done to check for the risk of certain conditions in the unborn baby such as Down syndrome. This is not the same as the hCG test described in this article.
The most important difference between urine tests and blood tests are that the blood test can tell you are pregnant earlier. A blood test may pick up hCG a week after conception, whereas urine tests may not pick up hCG until about a week later – that is, at the time of the first missed menstrual period.
As long as the tests are not done too early, you would normally get the same result whether you have a urine test at home, in the doctor’s office or in a laboratory, or a blood test done in the laboratory. However you may get different results if you do a urine test at home too early in the pregnancy, when it is still negative, then go to the doctor a few days later, when the result of a urine or blood test may be positive.
The urine test you do at home is convenient and private and avoids having a blood test, but it is important to follow the directions extremely carefully. The test should be done on a first morning urine sample before you have drunk a lot of fluid.
If a blood or urine test is done too early in pregnancy, when the hCG level in the body is still low, it is possible to get a false-negative result.
Urine tests may also give false-negative results if the urine is very dilute, so you should not drink a lot of fluid before collecting a urine sample for a pregnancy test.
Urine tests may give false-positive results if you have blood or protein in the urine, for example if you have a urinary tract infection. The urine test result can be wrong if you are taking certain medicines – some medicines cause a false-positive result and some a false-negative.
Home pregnancy tests should be confirmed by another test method and your doctor can organize confirmatory testing for you.
The blood test result can occasionally be wrong because of something in your bloodstream that interferes with the test, including certain types of antibodies that may give a false-positive result.
If you or your doctor thinks the blood or urine test result is wrong, you can ask the laboratory to measure your hCG level using a different test method.
It is also possible for a woman who is not pregnant to have hCG in her bloodstream at a level suggesting very early pregnancy. This tends to happen when women are tested at around the time of menopause. This is because hCG can be produced by the pituitary, a gland at the base of the brain. This is not harmful and does not indicate disease. To check whether or not the woman is pregnant, the test can be repeated in 2 days. If she is pregnant, the hCG should be approximately double what it was the first time it was tested. If she is not pregnant, the level shouldn’t change much.
An ectopic pregnancy is the condition when the fertilised egg implants somewhere other than in the womb. This is a serious condition needing immediate treatment. Women with ectopic pregnancies often have abdominal pain and uterine bleeding. A woman with an ectopic pregnancy will still have a positive pregnancy test because their placenta produces hCG. However, the level may be lower than in a healthy pregnancy.
To tell if a woman is pregnant or not, usually a urine test will give the answer, unless it is very early in the pregnancy (say, before the first missed period), in which case a blood test is needed. However, a blood test is needed if a doctor is concerned that something is going wrong the pregnancy, and he or she wants to know the actual amount of hCG in the blood. Blood tests for hCG are also needed to monitor people with trophoblastic disease (tumours developing from the placenta during pregnancy) or germ cell tumours (tumours developing from the sperm cells in males or egg cells in females).
It really depends what the hCG level in the blood was at the time of the miscarriage, but both urine and blood tests for hCG usually become negative within a few weeks of a miscarriage.
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