HCG is a hormone that is released by an embryo or fetus to let the women know that it is there and that she is pregnant. It is a critical hormone to that is used to make sure that the women does not reject the pregnancy. The test for HCG is done to determine whether or not the woman is pregnant. After it has been determined that a women is pregnant, then the HCG levels are some times not measured again. When they are measured again, it is not important whether they are high or low.
What is important is that the HCG levels are correct for your gestational age (age of your baby). In other words you don't want a high or low HCG level, you want a correct HCG level. This correct level is different depending on how long you have been pregnant. A level that is too low can mean that the baby is being miscarried. A level too high usually means that the date of conception is wrong (and the baby is actually older). In very rare cases, a high HCG level means that there is a tumor called a choriocarcinoma present in the uterus. The best physician for you to discuss this issue with is your OBGYN. I hope this helps answer your question, and good luck with your pregnancy!