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Pregnancy tests work one of two ways; either they measure whether or not there is hCG, or human chorionic gonadotropin, in your system, the way most home pregnancy tests work, or they measure how much hCG is present in the blood stream. This second type of test, called a quantitative blood test, is the one most commonly performed in doctor's offices. Depending on how far into the pregnancy you are, your doctor is looking for a minimum level, usually of 25mIU/ml by the five week mark.
After that first test, your doctor may request that you come in for additional blood tests. These tests are often necessary to monitor the progress of the pregnancy, especially in the first several weeks following conception. There are several reasons for multiple blood tests after pregnancy confirmation. The most common are a suspected multiple pregnancy, low initial numbers or slow rising numbers, a pregnancy achieved through fertility treatments, or an increased risk of miscarriage.
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In pregnancies with multiple implanted embryos, hCG levels are often higher than what would be expected with a single pregnancy. If your doctor suspects that you might be carrying more than one child, he might do second and third blood tests to monitor your levels until an ultrasound can be scheduled. Once a multiple pregnancy has been confirmed, a couple additional weekly blood tests may be used to monitor the pregnancy, because of the higher chance of miscarriage associated with twin or other higher level pregnancies. If the numbers don't continue to rise as expected, or if they start to fall, that could be an indication of a problem with one or more of the embryos.
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If your initial blood test results showed a lower than normal amount of hCG in your blood stream, your doctor may request a second test to make sure your numbers are rising as they should. Even if your first numbers were normal, your doctor may still schedule a second blood test to check how quickly they are increasing. If the numbers are rising slowly, your doctor will order regular blood tests to monitor your hCG levels. Slow rising numbers could mean nothing, but in some pregnancies, they are a sign of potential problems.
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If your pregnancy is the result of fertility treatments, especially if you became pregnant through the use of in vitro fertilization (IVF), your doctor will probably more closely monitor your hCG levels during the first several weeks of your pregnancy. There are two reasons for this. The first is because your doctor wants to make sure there are no complications in the early part of your pregnancy. The second reason is because many fertility treatments can increase the likelihood of a multiple pregnancy. Higher than normal test numbers might indicate the implantation of two or more embryos, and your doctor will want to closely monitor you until an ultrasound can be scheduled.
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Increased Chance of Miscarriage
According to American Pregnancy Association, between 10% and 25% of all pregnancies end in miscarriage, quite often before a woman even knows she is pregnant. If you have had one or more miscarriages prior to your current pregnancy, or if you have a high risk pregnancy, your doctor will most likely schedule multiple blood tests after pregnancy confirmation. Ideally, your test results will show that your numbers are increasing normally, and not leveling off or starting to drop. If your numbers aren't staying within a typical range, your doctor will order other tests in the attempt to diagnose any potential complications. These tests are necessary to catch a possible miscarriage early, which may give doctors a chance to treat the problem and save your pregnancy.
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Lab Tests Online: hCG: The Test. Accessed from: http://www.labtestsonline.org/understanding/analytes/hcg/test.html.
American Pregnancy Association: Human Chorionic Gonadotropin (hCG)-- The Pregnancy Hormone. Accessed from: http://www.americanpregnancy.org/duringpregnancy/hcglevels.html.
American Pregnancy Association: Miscarriage. Accessed from: http://www.americanpregnancy.org/pregnancycomplications/miscarriage.html.