Third Trimester Maternal Glucose Levels From Diurnal Profiles in Nondiabetic Pregnancies: Correlation With Sonographic Parameters of Fetal Growth: A response to Parretti et al. and Jovanovic

Parretti et al. (1) report an important study in which diurnal profiles performed on home blood-glucose monitoring were related to sonographic parameters of fetal growth. They imply in their introduction that the number of published studies concerning normoglycemia in nondiabetic pregnancies are few, a point that is also made by Jovanovic (2) in the accompanying editorial entitled “What is so bad about a big baby?”

Table 1 shows the diurnal plasma glucose profiles at 29 and 35 weeks gestation on identical diets from the subjects of our randomized controlled trial of low and relatively high dietary fiber diets given to women selected as being of normal weight and known to be nondiabetic (3). This study was published in 1983. In the original publication, two arms of the trial containing 12 and 13 subjects, respectively, were published separately for comparative purposes but have been combined for the purpose of this letter. Meals were taken as follows: breakfast before admission, midmorning snack at 1030 h, mid-day meal at 1200 h, midafternoon snack at 1515 h, evening meal at 1730 h, and supper at 2130 h. In these healthy women there was no deterioration in glucose homeostasis between 29 and 35 weeks gestation when studied on identical test meals (3).

In an earlier review (4) of factors possibly causative of macrosomia in the fetus of diabetic women, despite apparently good diabetic control, I raised three hypotheses that might explain how glucose could cause such an effect. The first hypothesis quoted Jovanovic et al. (5), stating that the postprandial glycemic peaks may be particularly high in diabetic pregnancy. However, these peaks may not be recorded on routine testing, a suggestion that has received further support in a recent publication from the North of England (6). The second hypothesis suggested that during overnight fasting, minor variations of fasting glycemia might have a disproportionate effect on fetal insulinization and growth. We provided some evidence to support this hypothesis in our own publication, again from nondiabetic women selected for body weight and studied in relation to fetal insulinization and neonatal anthropometry (7). This revealed a positive correlation between maternal fasting glucose in the third trimester and birth weight. The third hypothesis to explain macrosomia, despite good control in the diabetic mother, was that idiosyncracy might be operating in terms of transplacental glucose kinetics in individual maternal fetal pairs with different rates of glucose transfer, despite similar maternal glycemic levels. This latter hypothesis remains under investigation in our laboratory.