When that bump starts to grow, regular belly bars get uncomfortable as they have no flexibility, this belly bar resolves that issue for you. This belly button ring is designed for pregnancy with a flexible bio-flex clear bar with 5mm silver surgical steel balls ends.  But also is very common for other piercings which are sensitive to other materials such as tongue, nipple or inimate piercings. 
 
Specifications
Ball Material: Surgical Steel
Ball Size: 5mm
Bar Material: Bio-Flex