Low HCG, please adivse

37 years young, with a round of clomid, got a positive test on the day of my missed period. Went for blood testing x2 at 5 weeks and was told my HCG levels are slow to rise, not doubling as fast as the Dr would like. Going again tomorrow to confirm they are still rising. I am on the bottom of the required level ranges. Anyone have this before and what were outcomes or concerns?