Dietary treatment in gestational diabetes: Relation to birth weight Marianne Vestgaard, Allan Stubbe Christensen, Lone Viggers, Finn Friis Lauszus Dept. of Obstetrics and Nutrition, Herning Hospital, Denmark Corresponding author: Finn Friis Lauszus. Email: finlau@rm.dk   Background, Aim and Methods BACKGROUND Being large for gestational age (LGA) is a surrogate for many of the complications associated with gestational diabetes mellitus (GDM). LGA is complicated by a range of adverse pregnancy outcomes such as delivery injuries including increased frequency of induction of labour, shoulder dystocia and caesarean section. Furthermore LGA has been associated with metabolic abnormalities such as hypoglycemia and hyperbilirubinaemia. It is generally accepted that the cornerstone in the treatment of GDM is dietary modification, however the fully effect on birth weight is not known. MATERIALS AND METHODS We analyzed and compared the perinatal outcome of a two-year cohort of 189 women with GDM (excluding type 1 and 2 diabetes mellitus) with a control cohort of 254 women with a non-diabetic oral glucose tolerance test (OGTT). The women with GDM and the women with a non-diabetic OGTT had a similar background predisposition. The women with non-diabetic OGTT had meet the Danish criteria for screening with OGTT. The dietary habits in GDM were at first visit scored by a nutritionist into three groups: low, medium, and high adherence to current guidelines. Birth weight ratio was calculated as the observed birth weight divided by the expected birth weight for same gender and gestational age. AIM To evaluate the effect of dietary intervention on birth weight in GDM. Is it possible to reduce the frequency of macrosomia in GDM by dietary intervention? Results Data on 189 women with GDM at first visit at the nutritionist: The dietary intervention lasted 49 days (median) (3-191 days) 25 days of delay from diagnosis to intervention (median) (0-88 days) Birth weight decreased with 2.1g per day of treatment. High birth weight was associated with short duration of dietary intervention (p=0.02) No differences between women who received dietary intervention and women without dietary intervention regarding gestational age at delivery, frequency of elective and acute caesarean section, HbA1C and preeclampsia. 189 women with GDM 135 women received dietary intervention (17 treated with insulin) Low adherence to guidelines: 12% Medium adherence to guidelines: 40% High adherence to guidelines: 48% 54 women without dietary intervention (5 treated with insulin) Curve (life table) of birth weight ratio in three groups: women with hign adherence to guidelines (n=59), women with medium and low adherence to guidelines (n=76) and women with non-GDM (non-diabetic OGTT*) (n=254): There was a tendency towards a lower birth weight in women with high adherence compaired with women with low adherence (p=0.054) There was a tendency towards a lower birth weight in women with high adherence compaired with women with non-GDM (p=0.081) *Danish criteria for screening with OGTT during pregnancy: glucosuria, previous GDM, BMI > 27kg/m2, family history of diabetes and previous macrosomia. GDM is diagnosed if OGTT ≥ 9 mmol/l. Birth weight ratio Conclusions Dietary intervention reduces the fetal weight in women with GDM and the effect increases with length of treatment. The risk of large birth weight is reduced with adherence to current dietary guidelines. Macrosomia was as prominent in women with a non-diabetic OGTT as in the diagnosed GDM women. Dietary intervention may regulate GDM women's risk of macrosomia to similar levels as non-diabetic women.