Complex sentence: When women wear a bra with too big a band size, the band will ride up on their back which may place pressure on their vertebrae at the top of the back and the bottom of the neck (vertebrae C6 and C7), causing back pain.
Split into simpler sentences: When women wear a bra with too big a band size, the band will ride up on their back. This may cause pressure on their vertebrae at the top of the back and the bottom of the neck (vertebrae C6 and C7) and result in back pain.