Are early first trimester weights valid proxies for preconception weight?

Background: An accurate estimate of preconception weight is necessary for providing a gestational weight gain range based on the Institute of Medicine's guidelines; however, an accurate and proximal preconception weight is not available for most women. We examined the validity of first trimester weights for estimating preconception body mass index category. Methods: Under identical measurement conditions, preconception weight and two first trimester weights (i.e., 4-10 and 12 weeks gestation) were obtained (n = 43). Results: The 4-10 week and the 12 week weight correctly classified 95 and 91% women, respectively. Mean weight changes were relatively small overall (M = 0.74 +/- 1.99 kg at 4-10 weeks and M = 1.02 +/- 2.46 at 12 weeks). There was a significant difference in mean weight gain by body mass index category at 4-10 weeks (-0.09 +/- 1.86 kg for normal weight participants vs. 1.61 + 1.76 kg for overweight/obese participants, p = 0.01), but not at 12 weeks (0.53 +/- 2.29 kg for normal weight participants vs. 1.54 +/- 2.58 kg for overweight/obese participants). Conclusions: Assigning gestational weight gain guidelines based on an early first trimester weight resulted in 5-9% of women being misclassified depending on the gestational week the weight was obtained. Thus, most women are correctly classified based on a first trimester weight, particularly an early first trimester weight, although it is possible that modeling strategies could be developed to further improve estimates of preconception body mass index category.