Are you talking about the vertical crease or the horizontal crease.
The vertical crease is typically due to a natural thinner layer of fat beneath the skin. Insetting the umbilicus in a tightly repaired abdomen will oftentimes accentuate this vertical crease. Occasionally during surgery, patients have an excessively thick layer and a thin strip of fat is removed to create this crease. This is similar to abdominal etching.
The horizontal creases are due to natural creases in the rectus abdominis muscles called tendinous inscriptions. These are generally only visible in thin patients and can be more pronouced from repeated flexion (situps). These can be artifically created using abdominal etching.