It seems to be a mixed result. The problems that can occur during pregnancy with back pain involve the sacroiliac joints if one has had a fusion at L5/S1 due to the stresses on the spine being distributed to the SI joint. Normally the SI joint does not have much movement, but during pregnancy, a hormone called relaxin is secreted, causing an increase in mobility of the SI joint. This can result in some pain after pregnancy. The other site of pain is from the coccyx. After a fusion, the spine is rigid and the coccyx may lie in the way of the child coming through the birth canal...this occasionally results in pressure on the coccyx with dislocation or fracture.
Most of the time, if a person has not had fusion, and only had discectomies, the pregnancy is not as complicated. If a person has had a fusion, then some doctors would recommend a C-section to avoid the stresses on the SI joints and coccyx during the birthing process.
Also, epidural catheters for pain control during labor may not work as well if one has had spine surgery....