I would like to know what spirit I have to call on to help with my girlfriends pregnancy
Specifics of what part of her pregnancy you want to help or influence would be helpful otherwise you will get the generic list of health, healing, and support spirits. If that is what you are looking for Marbas and Raphael are very commonly recommended.
They given her a 20% chance to have our baby incompetent cervix
Alright well those two I mentioned should work well for that. If you have any interest or skill in it looking into qigong and healing work with that can be very effective as well.