I just have to say that being small is not really a medical reason to not be able to give birth vaginally.
I am 5 foot nothing and am a tiny frame and I had no trouble birthing my baby, my body knew exactly what it needed to do and it did it!
Women's bodies are designed to give birth this way (with some very rare exceptions).
Also, with the growth scans, I would take that with a grain of salt too, there are many threads around here about how far off the ultrasounds were.
Just my opinion and hope it helps you guys to have confidence in your bodies... you can do it!