Here is some very good information about the role of hCG in confirming a pregnancy:

"hCG is produced by the trophoblast cells of the placenta. hCG production starts at an early stage of development, just a few days after conception, before implantation in the uterus. hCG enters the maternal circulation almost immediately after implantation of the embryo (blastocyst) on about day 21 of the menstrual cycle. hCG is approximately eighty times more potent than its sister hormone produced by the pituitary gland, lutenizing hormone (LH). The tiny amount of hCG that enters the maternal circulation on day 21 (<5 mIU/ml in serum or plasma, while too small to be detected by pregnancy tests, is sufficient to stimulate the corpus luteum of the ovary to produce progesterone. The concentration of hCG in the circulation then rises in an exponential manner, doubling approximately every two days. By day 28 (first day of missed menses) the median hCG level in serum, plasma or urine is about 100 mIU/ml. This should be detectable by all pregnancy tests. At this time, the extremes of normal levels are approximately 5 to 450 mIU/ml in serum or plasma samples. For this reason we suggest waiting 3 days, until day 31 (31 days since start of last period, or 3 days after missing a period) before having an hCG test. This way one can tell if one has a true pregnancy (clinical pregnancy). Circulating hCG concentration continues to increase exponentially (doubling about every 2 days). It reaches a peak at about 10 week of gestation (since last menstrual period). At which time the median hCG concentration in serum or plasma samples is around 60,000 mIU/ml. "

This site show the ranges of hCG for each week of pregnancy. Obviously the higher on the scale the more likely the pregnancy will proceed, the lower, it will drop out.

What is beta hCG?