Macrosomia is the major cause of fetal morbidity and mortality in women with type 1 diabetes (formerly known as insulin-dependent diabetes). Even in the best obstetric centers, up to 40 percent of the infants born to these women have birth weights in excess of the 90th percentile. The reasons for this excessive birth weight may include complacency or fears that strict control of blood glucose concentrations could result in infants who are small for gestational age. Birth weight, however, does not provide the full picture of the fetal growth profile. Raychaudhuri and Maresh examined the relationship between glycemic control and infant birth weight in pregnant women with type 1 diabetes. Specifically, they monitored fetal growth rate in an attempt to determine the timing of accelerated growth in infants who were considered large for gestational age at birth.

Medical records of pregnant women with type 1 diabetes who delivered after at least 24 weeks' gestation were reviewed. All women entered the study before 12 weeks' gestation and had a first-trimester ultrasound examination. Subsequently, ultrasound examination for fetal abnormality was performed at 18 to 20 weeks, followed by repeated serial testing to assess growth from 24 until 36 to 38 weeks' gestation. In addition, fetal abdominal circumference was calculated at 20, 24, 28, 32 and 34 weeks. Glycemic control was estimated by obtaining glycosylated hemoglobin measurements at baseline and again every four weeks until 36 weeks' gestation. Women were taught to test their blood glucose levels at home and to record their blood glucose concentrations four times a day. Information about mode of delivery, gestational age, perinatal mortality and admission to the neonatal resuscitation unit also was reviewed.

Of the 82 infants delivered, 34 (41 percent) were large for gestational age, 42 (51 percent) were appropriate for gestational age, and six (7 percent) were small for gestational age. Mothers of large-for-gestational-age and appropriate-for-gestational-age infants did not differ significantly in any important maternal variable. At 20 weeks' gestation, the mean fetal abdominal circumference in large-for-gestational-age groups and appropriate-for-gestational-age groups did not differ; however, by 24 weeks and thereafter, there were significant differences between these groups. During weeks 18 to 24, mean maternal glycosylated hemoglobin concentrations were significantly higher in the large-for-gestational-age group. Mean maternal blood glucose concentrations also were higher in this group before 24 weeks. Between 28 and 32 weeks, glycemic control appeared to be comparable between groups. Mode of delivery did not differ significantly between groups, but women in the appropriate-for-gestational-age group tended to have more normal vaginal deliveries than women in the large-for-gestational-age group. Similar numbers of cesarean deliveries occurred between groups as well, but the decision for cesarean delivery was made more often during labor in women in the large-for-gestational-age group, suggesting slower progress.

The authors conclude that fetal growth potential appears to be determined by glycemic control before 20 weeks' gestation in women with type 1 diabetes. Despite appropriate glycemic control later in pregnancy, excessive growth and large-for-gestational-age deliveries still occurred. Interventions to reduce macrosomia in these infants should target glucose control during the first and early second trimesters.