Final answer:
The greatest risk to the fetus in a pregnancy with gestational diabetes that is not well controlled is the development of a macrosomic newborn, which refers to a baby that is significantly larger than normal, leading to potential delivery complications and long-term health issues.
Step-by-step explanation:
Among the complications associated with gestational diabetes mellitus (GDM), the greatest risk to the fetus if euglycemia is not maintained during pregnancy is a macrosomic newborn. Macrosomia refers to a condition where the fetus grows significantly larger than normal, often due to excess glucose from the mother's blood crossing the placenta and stimulating increased insulin and growth in the fetus. This can lead to complications during delivery, such as shoulder dystocia, and may necessitate a cesarean section.
Macrosomia can also have long-term health implications for the child, including an increased risk of developing metabolic syndrome and type 2 diabetes later in life. Other complications such as preterm birth and low birth weight are also concerns with GDM, but macrosomia is the most directly associated risk with uncontrolled blood sugar levels during pregnancy.