Human chorionic gonadotropin is a glycoprotein which is secreted by the syncytiotrophoblast of the placenta – the organ which nourishes the fetus.
This hormone helps to maintain the corpus luteum, which produces progesterone after the conception. Progesterone is necessary to sustain the pregnancy.
Seven genes located on the chromosome 19 encode the β-subunit of the human chorionic gonadotropin.
The elevated level of hCG may be detected in the blood 11 days after the fertilization and in the urine – after 12-14 days. Later the level of the hormone increases gradually until the 11th week of pregnancy and then decreases slowly.
The molecular weight of the protein is about 37.000-45.000 Da. Hormone comprises two subunits – β- and α-subunit.
The α-subunit (consist of 92 amino acids) is similar to luteinizing hormone, follicle-stimulating hormone and thyroid-stimulating hormone. The β-subunit (consists of 145 amino acids) is specific for hCG.
The half-life of hCG is 12-20 hours.
Reference levels of hCG in the serum
Nonpregnant females: 0 – 5 mIU/ml
Postmenopausal females: 0 – 8 mIU/mL
“Gray zone”, further examination is needed: 5-25 mIU/mL
Pregnancy: ˃25 mIU/mL
|3 weeks||5 – 50 mIU/mL|
|4 weeks||5 – 426 mIU/mL|
|5 weeks||18 – 7,340 mIU/mL|
|6 weeks||1,080 – 56,500 mIU/mL|
|7-8 weeks||7, 650 – 229,000 mIU/mL|
|9-12 weeks||25,700 – 288,000 mIU/mL|
|13-16 weeks||13,300 – 254,000 mIU/mL|
|17-24 weeks||4,060 – 165,400 mIU/mL|
|25-40 weeks||3,640 – 117,000 mIU/mL|
Measurement of the hCG helps to:
- Diagnose and date the pregnancy;
- Detect the ectopic pregnancy (the level of hCG in the blood increases slower than in normal pregnancy);
- Diagnose some types of cancer (molar pregnancy, germ cell tumors trophoblastic tumors) and monitor the treatment;
- Follow-up after the miscarriage and verify the need to perform dilatation and curettage (when the particles of the placenta remain after the abortion) – normally after the spontaneous abortion the level of the hCG should slowly return back to values in non-pregnant women. When the concentration of hCG fails to decrease dilatation and curettage should be considered;
- Predict the risk of congenital disorders and fetus anomalies such as Down syndrome and trisomy 18 – the hCG concentration in the serum is higher than the predictable gestational term.
The hCG urine test is the most simple method to detect the pregnancy. Various over-the-counter kits (home pregnancy tests) or point-of-care devices are available
This test is called qualitative as it may only either confirm or deny the presence of the hormone in the urine, but does not help to suggest the level of the protein in the serum However, the gonadotropin is detected in the urine when the concentration outreaches the threshold of 10-25 IU/L.
The urine test
The positive result of the test confirms the pregnancy. However, the negative test may not always be accurate, so it is recommended to repeat the test in a week. It is better to perform the test in the morning as the morning urine is more concentrated. The urine test has to be done by 10 days after a missed menstrual period.
A urine sample of about 60 mL should be collected in a sterile cup. The test strip has to be either immersed into the urine or the strip should be exposed to the urine stream while urinating. The method depends on the producer of the test, so a person should follow the prescript. The result will be ready in 5-10 minutes.
False-negative and false-positive results
Some drugs may influence the result of the urine test.
False-negative result may be caused by the diuretics (as they dilute the urine) and promethazine as well as the excessive drinking fluid before the test is done.
False-positive results may be caused by the anticonvulsants, hypnotics r tranquilizers. Some malignancies may increase the level of hCG in the serum and, therefore, in the urine.
To avoid the interference of the drug with the hormone the medication intake should be discontinued 3-4 days before the test.