Blood Test for Pregnancy

Blood and urine tests help confirm pregnancy. A blood test for pregnancy is also called serum test. Blood test can detect human chorionic gonadotropin at much lower levels when compared to a urine test. All tests work by detecting hCG which is made by the fertilized egg at first. The production of this bio chemical substance starts even before the fertilized egg is implanted in the uterus. The egg starts producing small amounts of hCG while traveling down the tube, towards the uterus but most tests are not sensitive enough to detect such a small amount of hCG. Once the fertilized egg gets implanted in the uterus and the placenta is formed, much larger amounts of hCG are produced by the placenta that can be detected with almost any of the pregnancy tests. Some tests can pick up small amounts of hCG while some tests are useful only when hCG is present in a large quantity. The tests which can detect small amounts of hCG, can thus detect a pregnancy earlier.

Blood Test for Confirmation of Pregnancy

A blood test is not done for routine pregnancy. It can detect hCG levels as low as 2-5 IU/L while most home urine tests detect levels of hCG as low as 100 IU/L (in fact, the range is 25-250 IU/L). Generally, blood pregnancy tests are only done for serious medical reasons, like in case of a suspected ectopic pregnancy. Even though a blood test can detect very low levels of hCG, hCG levels between 5-25 IU/L are considered inconclusive. Levels below 5 IU/L are truly negative and the result is not considered positive unless the hCG level is 25 IU/L or above. Since pregnancy blood tests are performed for serious medical conditions, over the counter kits for home use are not available.

The two types of blood tests are qualitative and quantitative blood pregnancy tests. A quantitative blood test shows the exact amount of hCG in the blood, while a qualitative hCG blood test gives a simple yes or no answer to the question "am I pregnant".

When Can You Take a Blood Test for Pregnancy

There is a steady increase in the hCG levels throughout the first ten weeks of pregnancy. The hCG levels double about every 2 days for the first 4 - 6 weeks. This is the time when women often suspect that they are pregnant and they might notice some early pregnancy symptoms. Since a pregnancy blood test can give a correct amount of hCG, it is okay to undergo a test during weeks 4 - 6. The test can confirm a pregnancy about 7 days after ovulation (which is about a week before your period is due). Blood test will yield a positive result if 5 IU/L of hCG is found in the blood. This level is usually reached shortly after conception.

The woman may repeat the test after a few days and compare the hCG values seen. HCG levels should show a rise and an upward pattern, as is expected in a normal pregnancy. Significant levels of hCG not following the normal upward path indicate an abnormal pregnancy, an impending miscarriage and sometimes an ectopic pregnancy. Remember, the hCG levels are only one clue; more tests need to be done to detect abnormalities. Physical examination of the woman, personal discussion and other tests will help reach a definitive diagnosis.

There are other reasons for doing a qualitative blood test for confirmation of pregnancy. Accuracy of the blood test and accuracy of home pregnancy test are said to be the same. But the cost of blood test is higher than the urine test as it has to be done in a doctor's office. The price may vary from doctor to doctor or from lab to lab. Blood test that helps detect pregnancy has a 98 to 99% accuracy rate. A urine test gives you instant results while you have to wait for some hours or some days to get the results of the blood test. Intake of some medications, for example, promethazine, diuretics, hypnotics, etc. can affect the results. So, consultation with doctor prior to the test is essential.

If the blood test for pregnancy is performed too early, it may give false negative results as there may not be enough hCG in the blood to detect a pregnancy. This means, the test can be negative, when you are actually pregnant. If a woman taking medicines containing hCG undergoes a blood test, the test will be positive even though the woman is not pregnant. This is known as false positive result. So it is necessary to inform your doctor about your routine medications, before a blood test is done, and the test has to be performed twice.