Final answer:
The biggest risk factor for shoulder dystocia in a pregnant woman with gestational diabetes is the gestational diabetes itself, due to its association with fetal macrosomia.
Step-by-step explanation:
The woman's biggest risk factor for shoulder dystocia is gestational diabetes. While family history, the height of her husband (i.e., potentially large fetal size), and parity can be contributors to the risk of shoulder dystocia, gestational diabetes is well-known to increase the risk. It can result in macrosomia, which is a condition where the baby is larger than normal. This increases the chance of complications during delivery, such as shoulder dystocia, where the baby's shoulder becomes lodged behind the mother's pelvic bone.