hCG is produced by the cells which will eventually become the placenta. Long before it is fully formed, the early placental tissue sends a message to the site of the ovarian follicle where the egg was released. This area is known as the corpus luteum and it plays a really important role in influencing the production of oestrogen and progesterone. These hormones are responsible for building up a rich vascular (bloody) lining in the walls of the uterus which will nurture and feed the developing embryo before the placenta has had a chance to form. Without this feedback loop occurring, the chances of the embryo surviving would be pretty slim.
Issues relating to the function of the corpus luteum are thought to account some women experiencing fertility problems.

But of course all of this upswing in hCG levels is occurring long before a woman has had her pregnancy confirmed. hCG starts being produced around a week after the egg has been released and then fertilised by the sperm. The woman may suspect she’s pregnant and be doing the date calculations, but it’s too early for there to be any definitive proof.

Where does hCG come from?