Human chorionic gonadotropin, hCG, is a protein hormone produced by the developing embryo during pregnancy. The hCG hormone is involved in the maintenance of the uterine lining and may also provide immune benefits early in pregnancy. Pregnancy tests use the presence of hCG to confirm that conception has occurred. This simple blood test can detect levels of hCG between eight and 11 days following ovulation. If hCG is not present or is produced at very low levels, the uterine lining will not be maintained and menstruation will begin.
Functions of hCG
The hCG hormone is required for the proper maintenance of the corpus luteum during pregnancy and for the secretion of progesterone, a hormone that is also involved in the development of the uterine lining. Progesterone allows sufficient blood flow to reach the uterus and sustain the developing embryo. The hormone may also play a role in immune functions. Rising levels of hCG have been implicated as the cause of morning sickness, which is commonly seen in the first trimester.
Testing
Levels of hCG can be measured through a blood or urine test. Blood tests are typically more sensitive than urine tests and can detect hCG levels as early as eight to 11 days after ovulation.
Normal Levels
The range of normal hCG levels is quite wide. In the majority of pregnancies, levels of hCG will double every 48 to 72 hours. As the pregnancy continues and levels of hCG continue to increase, it takes longer for serum hCG levels to double. Levels of hCG below 1,200 mIU/ml usually double every 48 to 72 hours while levels between 1,200 and 6,000 mIU/ml usually double every 72 to 96 hours. Once hCG levels reach 6,000 mIU/ml, doubling time increases to approximately four days. Pregnancy tests that detect hCG levels less than 5 mIU/ml are typically considered negative.
Low hCG Levels
In some pregnancies, lower hCG levels are not a cause for concern. A low hCG level can be the result of several causes and should be re-checked in 48 to 72 hours. Low levels may indicate a miscalculation of the day of conception, an ectopic pregnancy or a possible miscarriage.
High hCG Levels
Similar to low hCG levels, a high hCG level should be re-checked in 48 to 72 hours. High hCG levels can indicate multiple pregnancies, a miscalculation of the date of conception or a molar pregnancy. A molar pregnancy occurs when a genetic error occurs during fertilization. A lack of genetic information causes no embryo to form, but abnormal uterine tissue continues to develop. Since growing tissue is present, the placenta secretes hCG and a positive pregnancy test will result even though no embryo is present.
References
- "Human Reproduction"; Effect of Rising hCG Levels on the Human Corpus Luteum During Early Pregnancy; I. Y. Järvelä, A. Ruokonen, A. Tekay; July 2008
- "American Journal of Roentgenology"; Early Gestation: Correlation of hCG Levels and Sonographic Identification; D. A. Nyberg et al.; May 1985
