I wasn't weighed once during my pregnancy, by the hospital. However, in the week i gave birth i lost over 20kg, 3.5kg was baby and the majority of the rest was fluid. IF my BMI had sent me over the limit for a water birth i would be pee-ed off because it was due to excess fluid as a results of the pregnancy, not excess fat.

Good luck for the birth, and welcoming your new bub