Pregnancy with Monogenic Diabetes

Pregnancy is the situation where monogenic diabetes needs the most specialist input, and where the usual rules for your subtype may reverse.

In GCK-MODY, treatment depends on whether the baby has inherited the variant — which is unusual in medicine. A baby who has not inherited it responds to the mother's higher glucose by growing larger; one study found unaffected fetuses around 600 g heavier at birth, and insulin treatment is then indicated. A baby who has inherited it has the same higher set point and grows normally, and treating the mother can restrict that growth. In one registry, insulin treatment of affected fetuses lowered average birth weight from 3725 g to 2967 g — and carried a 23% incidence of severe hypoglycemia in the mothers.

So the decision is usually guided by fetal growth on ultrasound from the second trimester, and increasingly by testing fetal DNA in the mother's blood — though cost limits access, estimated around £2000 per patient.

In HNF4A-MODY, the newborn needs watching. Macrosomia occurred in 56% of carriers against 13% of unaffected family members, with birth weight around 790 g higher, and persistent newborn low blood sugar in 15.4% — so babies need blood glucose monitoring for at least 48 hours after delivery.

In HNF1A-MODY, birth weight is not significantly different, with growth assessed regularly from 28 weeks.

Sulfonylureas are generally not continued in pregnancy; insulin is the usual treatment.