HCG hormone levels are used to determine whether or not a woman is pregnant. In some cases the hCG level can be low, causing you to wonder if you're really pregnant.
HCG may be detected as early as 12 days after conception, but if implantation takes place late the levels can be too low to detect until later in the pregnancy.
The American Pregnancy Association identifies a range of normal hCG levels during pregnancy (see Resources below). Above 5 mIU indicates pregnancy, but if you are newly pregnant with levels below that threshold, blood and urine tests will be negative.
There are two kinds of blood tests to measure hCG. A qualitative test detects if hCG is present in the blood above a predetermined level (typically 5 mIU), while a qualitative test measures the exact amount of hCG. Low hCG levels can cause a false negative on a qualitative test.
One low hCG level is not enough to make any determinations. As the hCG level should double every two to three days, if a pregnancy test is questionable, repeating it a few days later can confirm whether the hormone is rising.
HCG levels actually decline after the first trimester of pregnancy. Low levels can be an indicator of miscalculation, indicating that you are not as far along as expected or that you are farther along than you thought.