measure the amount of a hormone called human chorionic gonadotrophin, otherwise known as hCG
or beta-hCG, in the body.
The hormone is secreted by cells of what will become the placenta as soon as the embroyo attaches itself to the wall of the uterus. It is not found in women who are not pregnant
, and can be detected 8 days after fertilisation. It allows the body to maintain ovulary secretions and prevent periods, so that the egg remains attached to the wall of the uterus.
The amount of hCG present in the system doubles every 2-3 days, reaches its highest level at around the 12th week of pregnancy, and then falls. It remains detectable
in the blood and urine throughout pregnancy.