Human chorionic gonadotropin, or hCG, is the pregnancy hormone and the first indicator of pregnancy. It is produced by chorionic cells from the moment of implantation. If the hormone is in the blood, it means that the embryo is fixed, and pregnancy can be confirmed [1].
When can blood tests determine pregnancy?
HCG can be detected in the mother's bloodstream as early as the eighth day after fertilization. At this point, the level will be about 10 mlU / ml [1], but this figure can vary significantly.
How widely do the indicators differ, and what does the difference depend on?
Commercial labs use many combinations of antibodies to test hCG levels, which leads to a wide range of results. Indicators obtained on the same day from the same woman but in different labs can differ 50 times [2].
How is hCG measured, and what is its importance?
HCG levels are usually expressed in milli-international units per millimeter (mIU/mm) or milli-international units per milliliter (mIU/ml).
To make sure the pregnancy is developing normally, it is more important to check how levels compare over time than to see if a certain level is reached at a given moment during the pregnancy.
Normally, hCG levels rise by 50% every 24 hours for the first eight weeks. The peak is reached at 10–11 weeks of gestation, and then the level decreases gently until about week 16, when it reaches a plateau; with no significant change until the birth [2].
Therefore, if you took the test very early (even before a delay in menstruation), it makes sense to repeat it in the fifth or eighth week of pregnancy and possibly between weeks 11 and 13, along with an ultrasound.
What if the level of hCG appears higher or lower than normal in a test taken later?
If the hCG level reaches a plateau before eight weeks or rises very slowly, then a missed abortion or ectopic pregnancy may be the cause [2]. Of course, a diagnosis can not be made based on these tests alone, and an ultrasound is needed for confirmation. This is especially important when comparing results obtained from different labs [3]. A high level of hCG, that exceeds the reference values by 30% or more, may indicate a twin pregnancy [4].
