Human chorionic gonadotropin (hCG) is a small glycoprotein hormone produced by cells that form the placenta during pregnancy. The hormone is composed of two different subunits, alpha (α) subunit and beta (β) subunit. The alpha subunit is common to all members of the glycoprotein-hormone family, whereas the beta subunit is unique to hCG and confers its biological activity. All pregnancy tests are based on the beta subunit.
There are many different forms of hCG based on the degree of glycosylation and different proteolytic forms. Not all hCG tests detect or measure all these forms of hCG equally. Although the measurement of hCG is primarily performed to detect pregnancy, hCG is also utilized for other diagnostic purposes, such as detecting miscarriage and abnormal growth in certain types of cancer.
Levels of hCG rise rapidly during the first 10 weeks of pregnancy. The hormone signals the body to produce more estrogen and progesterone that thickens the uterine lining to maintain the pregnancy and stop menstruation.
In pregnancy, hCG levels that are considered lower or higher than normal range may not be cause for concern; however, low levels may indicate potential pregnancy loss, decreased fetal growth or lower birth weight. Higher levels can indicate multiple birth, gestational trophoblastic disease or preeclampsia.
Outside of pregnancy, hCG occurs naturally in both males and females, and levels of hCG are generally not detectable in healthy individuals. When hCG levels rise above normal levels in nonpregnant individuals, it can be a sign of:
- Cancer (liver, stomach, pancreas, lung, breast, skin)
- Tumors of the ovary or testicles
- Inflammatory bowel disease
- Cirrhosis
- Stomach ulcers
- Reduced sperm count
- Low testosterone
hCG injections may be prescribed to help with fertility issues in males and females. Due to the role of hCG in pregnancy, research studies have been performed to determine whether hCG treatment may help prevent miscarriage for women who have experienced recurrent pregnancy loss. According to the American College of Obstetricians and Gynecologists, hCG has not been proven to prevent miscarriage.
Pregnancy tests can be in the form of either urine tests or blood tests, however the most widely-used tests still utilize urine samples. Urine pregnancy tests are accurate, economical and are nearly as accurate as blood pregnancy tests. Blood tests are typically able to detect lower levels of hCG that those of urine tests, and can be used to determine gestational age.
There are two types of blood tests for the detection of hCG:
- Qualitative – to determine the presence or absence of hCG
- Quantitative – to measure how much hCG is present
Pregnancy tests can detect a pregnancy as early as 10 days after conception, especially if a first morning urine sample is used to perform the test. False-positive tests can occur if a woman is receiving hCG injections to treat infertility. False negative results can occur if the test is used too early in the pregnancy. A false negative test can also occur late in pregnancy due to very high hCG levels that can cause interference in the test.
