Too small for baby?

Well I'm 16 weeks pregnant and my little one is growing so fast, and that's what makes me nervous. My husband is 6'3 and I am 5'1 and every week little one gets bigger I don't see my pelvis widening out that much. Is anyone out there tiny and able to have a vaginal birth? This is my first and I'm really nervous my pelvis is too small. This happened to a friend of mine her husband was very tall and she is my size and she had to have a c section ( her baby was exceptionally big though ). I'm so very nervous because I have a very strict birth plan that I would like to stick to. Please somebody ease my anxiety!