You need to be careful of two things. First, you want the comfort level of knowing that the company producing the prototype for you is reputable and willing to agree to strict confidentiality and nondisclosure restrictions. Second, you want to be sure that any improvements that the company makes to the product remain your intellectual property.
Otherwise, you could find yourself midway through a patent application, only to find you have an unintended co-inventor. Ideally, you'll want to speak with a patent attorney in your area from the outset so that you can map out a strategy for how best to proceed.