What kind of lens for outdoor maternity shoot?

I have a nikon and have a 50mm. I have also rented a 24-70mm. I have a family member that wants to do maternity pics outside. Rustic theme. Does anyone know which lens would work the best for a maternity shoot?

If you can only take one lens, obviously use the 24-70.

Just as important - take an external flash to maximize your control over exposure. You ate very likely to need a bit if fill flash.