ANSWER:
Both you and your grandmother are wrong!
Reciting this Ayah for child birth and reciting it for a specific number of times is not permissible as it’s an innovation. And what your grandmother told you about it being very strong and stuff is totally baseless and fabricated!
There are no special dhikr to be recited during pregnancy. Just make general dua for easy pregnancy and a healthy and righteous child who would be the coolness of your eyes.