I can give you a great example as to why your DR should track down to zero:
My sister was told that she was having a m/c. She had previously had an ectopic pregnancy (the Hcg levels in ectopic usually do not double every three days as in a normal pregnancy, and can also drop, and increase), and she was at risk for another (having an ectopic puts you at an increased risk for another ectopic). So, when she became pregnant again, she had an Hcg, and her numbers were low for how along she was supposed to be. They did another Hcg, and it was still low for dates. She started bleeding, and she was told that she was m/c. She bled for a couple days, and then stopped. She then developed pain or her right side. She had anotehr Hcg, and her numbers had dropped significantly....but, not to zero. Her DR did not tell her to get another Hcg. She started bleeding off and on, and the pain on her right side increased, espesially during a BM (which is common in an ectopic pregnancy). She went to the drop-in clinic, and he said it was "indigestion". Later that same day, she went to the ER because she passed out from the pain, and the DR there told her that it was probably a uterus infection and prescribed her antibiotics. But, "just to be sure", he had said, he would "take another Hcg result". It had been weeks since her so-called m/c, so her Hcg level should have been at zero, espesially since it was already falling. Well, it came back in the thousands. At this point, my sister started hemoraging in the ER, and was raced into immediate surgery. She lost her other fallopian tube (she lost the one befroe from her previous ectopic), one ovary, and her uterus. She had to have 4 blood transfusions, and almost had to have another surgery from internal bleeding. She just about died. It was awful!
If they had followed her numbers right down to zero, it wouldn't have gone that far. When I had my m/c, I demanded my numbers be tracked right down to zero.