Birth Size Distribution in 3,705 Infants Born to Mothers With Type 1 Diabetes

OBJECTIVE To characterize birth size distribution in infants born to mothers with type 1 diabetes. In particular, the relationship between birth weight (BW) and length (BL) was studied because it may provide information on different causal pathways of fetal macrosomia commonly seen in diabetic pregnancies. RESEARCH DESIGN AND METHODS This was a population-based cohort study of 3,705 infants of type 1 diabetic mothers (1,876 boys), with a gestational age of 28-43 weeks, born in Sweden between 1998 and 2007. BW and BL were retrieved from the Medical Birth Registry and expressed as SD scores (SDS). Ponderal index (PI) was calculated as BW in g/length in cm³. A BW >90th and a PI ≤ 90th percentile was defined as proportionate large-for-gestational age (LGA), whereas if both BW and PI > 90th percentile, the infant was categorized as disproportionately large. Values are mean (SD). RESULTS The BW distribution for offspring of type 1 diabetic mothers was bell-shaped, significantly broader, and markedly shifted to the right (BWSDS: 1.27 [1.48]) of the reference. Of the infants born to diabetic mothers, 47% were LGA, and among them, 46% were disproportionately large compared with 35% in nondiabetic LGA infants (P < 0.001). Female offspring of type 1 diabetic mothers had significantly higher BWSDS than males (1.34 vs. 1.20, P < 0.01), and preterm infants had higher BWSDS than term infants (1.41 vs. 1.23, P < 0.01) CONCLUSIONS Fetal macrosomia in type 1 diabetic pregnancies is due to a right-shift and broadening of the entire BW distribution. The large number of disproportionate LGA infants born to type 1 diabetic mothers suggests an underlying metabolic problem. Fetal macrosomia was more pronounced in preterm and female offspring of type 1 diabetic mothers.