Leaving my son alone with his Grandma for the first time

Hes 1 year and has never been with anyone except his Dad or I (I'm a SAHM). I have an US and appointment with my OB after and my husband would like to attend and its just going to take to long for my son to come and be happy during. So my MIL is going to watch our son. She's great with him the only problem is that she disrespects rules that we have put in place with our son. I know as soon as I walk out the door all of my rules and my sons schedule leaves with me. I'm an anxious mess. She is the only one available to watch him while in in the hospital for birth so this is needed to get him used to being without Mom and Dad but I'm freaking out! Ugh this is gonna be the longest 2 1/2-3 hours of my life! How did your first time leaving your children with a family member go? Maybe help ease my mind?