This is the last injectable drug of the treatment. It is administered in a single dose at a specific time (around 36-38 hours prior to egg collection) when the follicular development is deemed satisfactory to proceed with the egg collection.

hCG is a hormone that is normally secreted during the placenta development. Its measurement is used for the detection of pregnancy. However, an area of the hCG molecule is very similar to the LH molecule and it is therefore possible for the hCG to bind to the LH receptors and mimic its action. This means that the administration of hCG in a single powerful dose can "trick" the ovary and induces follicle rapture around within the next 36 hours (please note that the normal hormonal signal of ovulation in the human body is LH, which is released in a large surge when oestradiol levels go over a certain point).

hCG is available in a pharmaceutical form for intramuscular injection with the commercial names Pregnyl or Profasi (urinary hCG). During the last few years there is also a recombinant form of hCG (r-hCG), with the commercial name Ovitrelle. Ovitrelle is available in a pharmaceutical form for subcutaneous injection due to its high purity.