When a large (11 pound), macrosomic, plethoric (resembling the infant of a diabetic mother) baby is born to a mother without diabetes, what is the reason?

Most instances of macrosomia are related to hyperinsulinism at some stage in fetal life. Whilst the most common occurrence is in association with overt maternal diabetes, hyperinsulinism sufficient to cause macrosomia may occur in mothers who are unusually tall or heavy before pregnancy, who have excessive weight gain during pregnancy, or who have a mild degree of gestational glucose intolerance that is short of the defined limits for gestational diabetes.

Much more rare are states that may be associated with a range of congenital abnormalities like Beckwith syndrome or the Simpson-Golabi-Behmel syndrome. Diagnosis and treatment, if needed, must rest with the obstetrician or pediatrician.

DOB
Additional comments from Dr. Bill Jones:

In addition, I would suggest early screening for diabetes in the next pregnancy as well as serial ultrasound examinations to follow fetal growth. Delivery can be very complicated for the macrosomic fetus. If the large size is due to a specific syndrome (Beckwith-Wiedemann for example) then genetic testing can be done.

OWJ

[Editor’s comment: There is a very specific medical jargon phrase that is mentioned in the question: infant of a diabetic mother (or “IDM”). The phrase “infant of a diabetic mother” is not used to describe all babies born of diabetic moms as might be supposed, but is reserved for use to describe a typical appearance of a newborn baby who is big (“macrosomia”), flabby, and reddish (“plethoric”), usually as a direct result of mom’s inadequate control of her blood sugar (for whatever reason) during the latter half of the pregnancy.

WWQ]