Human chorionic gonadotropin, or hCG, is a hormone produced during pregnancy. It is detectable both in the urine and in the blood. Most home pregnancy tests are designed to detect hCG in the urine. Serum hCG, or quantitative hCG, tests determine the exact amount of pregnancy hormone present in the blood.
The Test
To test serum hCG levels, a sample of blood must be taken. If a woman is not pregnant, there will be no detectable amounts of hCG in her blood. According to LabTestsOnline, in early pregnancy, a woman's hCG level doubles every two to three days due to the rapid growth of the placenta. Women with failing pregnancies or ectopic pregnancies, which occur when the egg implants somewhere other than the uterus, will see a longer doubling period or even a decline in hCG levels.
Positive Pregnancy Test
The length of pregnancy is measured in weeks starting with the first week following the last menstrual period (LMP). A pregnancy test is considered positive if the test detects at least 5mIU/ml (milli-international units per milliliter). According to the American Pregnancy Association, hCG levels reach this milestone approximately three weeks after the last menstrual period.
The First Trimester
Normal serum hCG results vary from woman to woman. Robert Green, M.D., FACOG advises that most hCG results fall into certain ranges. A woman who is at four weeks gestation can expect her hCG levels to be anywhere from 4 to 425mIU/ml. As pregnancy progresses, the range of normal results expands. At eight weeks gestation a normal amount of serum hCG is 7,650 to 229,000mIU/ml. At 12 weeks normal hCG levels can climb as high as 288,000mIU/ml before beginning to decline in the second trimester.
The Second Trimester
Week 13 of pregnancy marks the beginning of the second trimester. At this point hCG levels begin to decline. This is also the point when morning sickness typically is abated for many expectant mothers. The placenta, now formed, decreases the amount of hormone it produces, and a woman between 13 and 16 weeks gestation can expect her hCG levels to range from 13,300 to 254,000 mIU/ml. At 17 to 24 weeks the hCG levels could fall as low as 4,060 or be as high as 165,400mIU/ml. HCG levels continue to decline in the third trimester and will fade completely following delivery.
Lower than Normal Results
Serum hCG results that fall out of the normal range could indicate a number of things. However, one result is insufficient to make a diagnosis. Another serum hCG test will be ordered in two to three days to see how the levels change over time. However, according to the American Pregnancy Association, lower than expected hCG results are typically attributed to one of three factors: a miscalculation of pregnancy dates, possible miscarriage or an ectopic pregnancy.
Higher than Normal Results
As with low results, higher than expected results could also mean a miscalculation of pregnancy dates. In this case, the pregnancy could be farther along than originally thought. Mothers carrying twins or more will also expect to experience higher than average serum hCG levels. Another possible reason for hCG levels to rise above normal is a molar pregnancy. The American Pregnancy Association explains a molar pregnancy is a pregnancy in which the placenta and the embryo, if present, do not develop properly. Placental tissue grows quickly in the early phases of a molar pregnancy, resulting in extremely elevated levels of serum hCG. This type of pregnancy is rare, occurring in only one out of every 1,000 confirmed pregnancies in the U.S.
