HCG is a hormone that is produced specifically in the trophoblast (the structure that will later give rise to the placenta) and that maintains the corpus luteum so that it continues to produce hormones that, in turn, will maintain gestation. It is made up of two subunits, the alpha subunit which is common to other hormones produced by the pituitary gland and the beta subunit which is specific to HCG.
As gestation progresses, the levels of this hormone increase. Although there are large variations from one week to the next, and even from one day to the next, there are approximate values for each period of pregnancy. Therefore, in early pregnancy, a low Beta HCG may be indicative of a pregnancy that is not progressing properly.
However, as mentioned above, given the high variability in the concentrations of this hormone and its daily changes, the determination of a single value is not predictive of the evolution of the pregnancy. If there are doubts as to the normality of its values, we usually determine it serially (approximately every 48 hours) to see if the HCG levels increase adequately.