Why do you give a pregnant patient 4-6 glasses of water before an Ultrasound? 

Because ultrasound travels through water.

Explanation:

This is not common practice in ERs and I would not recommend giving a PT anything PO unless safe to do so.

That being said, I'm sure whoever asked you this question wants you to infer that Ultrasound equipment uses sound wave resonance to visualize what is occurring through barriers. This is why they use the gel with the probe so as to allow better transfer of the sound waves.

it's a nutrition food for pregnant mothers so they unborn child can get its nutrition that it needs to grow in to a healthy baby.

explanation:

and the mother is getting all of her nutrition and her calcium and vitamins .