Dramatic changes in insulin sensitivity may occur in insulin-dependent diabetics at the time of delivery. Once active labour has started, insulin requirements fall. After delivery, once the placenta and its hormonal products have been removed, there is a further rapid reduction in insulin requirement. Indeed, immediately after delivery, insulin requirements may fall below pre-pregnancy values.
During labour the simplest scheme is to use a constant infusion of 10% glucose at a rate of 1 L every 8 hours. An independent insulin infusion of human soluble insulin, initially at 1 unit/h, is also given; this is subsequently adjusted on the basis of hourly bedside blood glucose. This system may be used irrespective of the last subcutaneous insulin dose, but where induction of labour or caesarean section is planned it is best started at breakfast time after a bedtime injection of isophane insulin. As soon as the infant is delivered, the insulin infusion must be reduced or, in women with gestational diabetes, stopped altogether. The glucose infusion is continued until the next meal in patients who had vaginal deliveries or until a normal diet is resumed in those delivered by caesarean section. The pre-pregnancy insulin doses should be resumed at this time and adjusted according to the blood sugar levels. An additional 40-50 g carbohydrate, relative to the pre-pregnancy dietary intake, is generally recommended during lactation. Women should also be warned about the potential risk of hypoglycaemia whilst feeding, especially in the middle of the night. They may need advice on appropriate snacks or fluids that contain carbohydrate. Oral hypoglycaemic agents, where they were being used before pregnancy, are probably best avoided. Small quantities of sulphonylureas are secreted into breast milk and therefore can theoretically induce hypoglycaemia in the infant. This is probably of significance only with the longer acting sulphonylureas such as chlorpropamide. Metformin is not recommended for use in lactation. However, there is no evidence of harm for the infant from the small amount of metformin that is secreted into breast milk. Infant exposure to metformin can be minimised by breastfeeding just before taking the dose and by avoiding feeding for at least 2-3 hours after taking the dose. It has been suggested that prophylactic antibiotics should be given after operative deliveries to offset the increased risk of wound infection in women with diabetes.