bhcg rising rate

So I've (31F) got a positive for my first IUI. I've had 3 blood draws over the last week to confirm doubling rates. My clinic seems to be very cautious and telling me that they're not doubling as quickly as the doctor would like. However, all my research indicates I'm right on track and there shouldn't be any concerns. Do you find that your fertility clinic is just cautious when explaining your labs? They had me schedule an ultrasound once they confirmed I got over 200 bhcg. I go in for an ultrasound Wednesday morning (5 weeks, 4 days).

My blood draws have been:

3w6d bhcg 57, progesterone 51

4w2d bhcg 172

4w5days bhcg 329