i agree with mags - i think birth weight can also depend on race/ethnicity and also the size of mom and dad (irrespective of heritage). both DH and i are relatively small (i am 5'1" and was 118 pre-pregnancy; DH is 5'7" and 120-ish). i gained 14 pounds with my first and he was 6 lbs. 4 oz. at birth.
a very recent cross-sectional study indicates that women who gained more weight than was recommended during pregnancy had a 20% greater chance of having babies >= 4000 grams (or about 8 lbs 13 oz). this was true for women irrespective of their pre-pregnancy BMI (underweight, normal, overweight, obese, morbidly obese). cross-sectional studies aren't the optimal study design, but they can suggest further research areas using stronger designs.
i also agree with the sentiment that what we eat matters too. i find that friends who have done the bradley method tend to have larger babies (perhaps b/c of the high protein intake). we should also consider that nowadays the animals consumed are pumped full of growth hormones, which inevitably end up in the bodies of those who consume them. although i'm not aware of comprehensive studies done in this area, i would imagine that such hormones can and do affect the birth weight of our babies.
here's the abstract:
The effect of gestational weight gain by body mass index on maternal and neonatal outcomes.
Crane JM, White J, Murphy P, Burrage L, Hutchens D.
Department of Obstetrics and Gynecology, Eastern Health, Memorial University, St. John's NL.
OBJECTIVE: To evaluate the effects of gestational weight gain on maternal and neonatal outcomes in different body mass index (BMI) classes. METHODS: We compared maternal and neonatal outcomes based on gestational weight gain in underweight, normal weight, overweight, obese, and morbidly obese (BMI>or=40.00) women. The study group was a population-based cohort of women with singleton gestations who delivered between April 1, 2001, and March 31, 2007, drawn from the Newfoundland and Labrador Provincial Perinatal Program Database. Univariate analyses and multivariate logistic regression analyses (controlling for maternal age, parity, smoking status, partnered status, and gestational age) were performed and odds ratios (ORs) were calculated. RESULTS: Only 30.6% of women gained the recommended amount of weight during pregnancy; 52.3% of women gained more than recommended, and 17.1% gained less than recommended. In women with normal pre-pregnancy BMI, excess weight gain was associated with increased rates of gestational hypertension (OR 1.27; 95% CI 1.08-1.49), augmentation of labour (OR 1.09; 95% CI 1.01-1.18), and birth weight>or=4000 g (OR 1.21; 95% CI 1.10-1.34). In overweight women, excess weight gain was associated with increased rates of gestational hypertension (OR 1.31; 95% CI 1.10-1.55) and birth weight>or=4000 g (OR 1.30; 95% CI 1.15-1.47). In women who were obese or morbidly obese, excess weight gain was associated with increased rates of birth weight>or=4000 g (OR 1.20; 95% CI 1.07-1.34) and neonatal metabolic abnormality (OR 1.31; 95% CI 1.00-1.70). In morbidly obese women, poor weight gain was associated with less use of epidural analgesia (OR 0.34; 95% CI 0.12-0.95). In women who were of normal weight, overweight, or obese, the rate of adverse outcome (Caesarean section, gestational hypertension, birth weight<2500 g or birth weight>or=4000 g) was lower in women with recommended weight gain than in those with excess weight gain. Adverse outcomes were reduced in nulliparous morbidly obese women who had poor weight gain (OR 0.18; 95% CI 0.04-0.83). CONCLUSION: The effects of gestational weight gain on pregnancy outcome depend on the woman's pre-pregnancy BMI. Pregnancy weight gains of 6.7-11.2 kg (15-25 lb) in overweight and obese women, and less than 6.7 kg (15 lb) in morbidly obese women are associated with a reduction in the risk of adverse outcome.