During pregnancy, most women develop a split in their abdominal muscles. Sit-ups increase the abdominal pressure and encourage the split between the abdominal muscles, making the recovery of these muscles after childbirth slower and problematic.
Sit ups do NOT strengthen the core stabilisers. It does NOT work the core stabilisers, the transversus abdominus and multifidus. These muscles need to be trained to help with the recovery of the split in the abdominal muscles.
Sit ups is a dynamic exercise, specifically training the prime movers which often inhibits the activity of the core stabilisers from working. Without the strong base of core stability, sit ups can cause back pain, as it puts extra pressure on the discs of the lower back. It also increases the risk of injuring the discs and causing long term lower back pain.