What one expert should we see if they suspect abdominal wall defect in our fetus?

Perinatologist. First start with a perinatologist (maternal-fetal medicine specialist). They'll screen the infant for other birth defects and/or chromosomal problems. Depending on what they find, they'll coordinate delivery with a pediatric surgeon if needed & a neonatologist. But first start with a good mfm specialist. Best wishes to you.
MFM, then fetal cntr. See an experienced maternal-fetal medicine specialist to assess your fetus sonographically and then (depending on the mfm findings) seek referral to one of the nation's few fetal centers for multidisciplinary aproach to your baby's problems. An example: http://women.Texaschildrens.Org/fetalcenter/.