What is the strongest predictor of birth weight: Gestational age, hbalc, maternal weight, weight gain, or birth weight of sibling?

Aim: To assess the effect of various maternal characteristics in diabetic pregnancies on birthweight (BW) taking into account birthweight of an elderly sibling.  Method: We identified all pregnant diabetic women in North Jutland County. Birthweight and certain maternal characteristics including HbA1c values after 20th gestational week were collected. Multiple regression models were fitted to assess the effect various predictors of birthweight in both entire cohort (n=501) and in a subset with available weight of sibling (n=139). Sibling weight was calculated as relative to expected weight adjusted for age and sex using a Danish reference. E.g. an observed sibling weight of 3800 g with expected BW 3400 g predicts 11.8% extra weight equal to 134 grams (114x11.8) and one extra gestational day predicts an additional weight of 27 grams.  Results: The effects in terms of additional grams in BW for various increments in each of the 9 variables are seen in the table. All significant values are in bold face.  Conclusion: Weight of sibling is a very strong predictor of birthweight attenuating the predictive power of all other variablesapart from gestational age.