The cause for the cracks can vary. Certainly, the mix is 1 cause. Settling, improper reinforcing, poor footings, among others, can cause cracking. THe question is - How much larger will they grow?
First, do you have a New Home Warranty from the Builder? If not, see about getting one. Usually, 1 is offered for the first year at no charge. Option 2, hire yourself a NEW Home Inspector. Visit http://www.ashi.com for some help. Don't hire one that's been referred to you by the Builder, the Realtor, the Finance folks, or the like. Preferably, find on that works OUTSIDE your immediate area. These types of inspectors do exist. Just be SURE of the background of your HI. Follow the 'guidelines' from ASHI, as a start.
In ANY case, you should document your findings. I'd take pictures and start writing notes. I'd take notes of the date and time and the name of the person(s) you talk to about the 'problems' w/your home. Long story short - WHen it comes right down to it, and you're the Judge, what would YOU want to see as proof that you were 'communicating' w/the Builder? You should follow up EACH conversation w/a Certified Letter, Return Receipt. (Try to keep these at a minimum. You don't want to piss off the Builder.) Try to work w/him.
SOooo, depending on what the cracks do from here, that will tell what problems you will have in the future. THey could be none, to many. Others may offer more advice, so stick around.
My best to ya and hope this helps.
Jay J -Moderator
PS: If you haven't hired a Real Estate Attorney, they're very inexpensive. The Attorney should be reviewing EVERYTHING before you sign it!!!