Human chorionic gonadotrophin; hCG
Human chorionic gonadotrophin (hCG) is a hormone produced by the cells that surround the growing human embryo that go on to form the placenta (called trophoblasts). Human chorionic gonadotrophin can be detected in the urine from 7-9 days after fertilisation as the embryo attaches and implants in the womb; it forms the basis of most over-the-counter and hospital pregnancy tests (see photo).
During the menstrual cycle, when an egg is released from the ovary at ovulation, the remnants of the ovarian follicle (which enclosed the egg) form the corpus luteum, which produces the hormone progesterone (and other hormones such as oestradiol).
If the egg is fertilised and implants, it produces hCG, which maintains survival of the corpus luteum. The corpus luteum will then continue to produce progesterone (as well as oestradiol and other hormones), which acts on the lining of the womb (endometrium, also called the ‘decidua’ during pregnancy) to maintain the pregnancy.
If implantation does not occur, hCG levels fall, and the corpus luteum expires and stops producing progesterone. This fall in progesterone levels leads to shedding of the lining of the womb (menstruation) and through loss of the negative feedback, leads to an increase in follicle stimulating hormone (and to a lesser extent luteinising hormone) from the pituitary gland to initiate the next menstrual cycle.
Human chorionic gonadotrophin is produced by the trophoblast cells which surround the developing embryo and go on to form the placenta. The amount of human chorionic gonadotrophin in the bloodstream doubles every 2-3 days as development of the embryo and placenta continue, and levels peak at around 6-8 weeks of pregnancy. Following this peak, levels of human chorionic gonadotrophin plateau (although they remain detectable throughout pregnancy). Once the placenta is established, it becomes the main source of progesterone production (from around week 8-12 of pregnancy), and human chorionic gonadotrophin is no longer required to maintain the function of the corpus luteum. However, human chorionic gonadotrophin may have additional beneficial effects in the latter stages of pregnancy which are currently under investigation by researchers.
There is no strong evidence that high levels of human chorionic gonadotrophin cause direct negative consequences. Very high levels of human chorionic gonadotrophin are rare but can indicate hyper-proliferation of the placenta (also referred to as hydatidiform moles or molar pregnancies), which can in some people lead to a form of cancer (choriocarcinoma). Rarely, levels of human chorionic gonadotrophin may also be elevated in association with some non-pregnancy related cancers (e.g. kidney, breast, lung and gastrointestinal tract). In such cases, levels of human chorionic gonadotrophin in the blood/urine can serve as a tumour marker.
In pregnancy, a link between high levels of human chorionic gonadotrophin and occurrence of Down’s syndrome has also been suggested. Studies have shown that the levels of human chorionic gonadotrophin in a Down’s syndrome pregnancy are approximately twice that of an unaffected pregnancy. However, it is important to note that high levels of human chorionic gonadotrophin do not cause Down’s syndrome (rather it is caused by an extra chromosome at position 21); further research is needed to investigate this link.
Low levels of human chorionic gonadotrophin can indicate a failing pregnancy. Reduced levels of human chorionic gonadotrophin are often observed in ectopic pregnancies (where the embryo implants outside of the uterus) or in miscarriages.
Human chorionic gonadotrophin can be used to treat men with hypogonadism and infertility. It acts in a similar way to luteinising hormone (LH), stimulating testosterone and sperm production in the testes. It can be administered via intramuscular or subcutaneous injection. Side effects can include swelling of the testes, groin pain and behavioural change.
Last reviewed: Sep 2021