Fibrous cartilaginous discs which sit in between the vertebrae in the spine.  If the discs become damaged they can bulge into the wall of the spine and put pressure on the roots of the sciatic nerve.  A Herniated disc is where the gel inside is expelled due to a trauma.  This then puts pressure n the nerve roots.  The lumbar spine is most prone area to disc issues.

The whole point of our pre and post natal classes is to create comfort at each stage of pregnancy.  Safe postures generally are Cat, Downdog modified (at wall), back extension (in thunderbolt hands behind leaning back length in the front)

There be cases where a disc issue was present before the pregnancy where by a back bend will aggravate the disc therefore a forward folding asana is needed. Please always check with the mama.