The purpose of this study was to examine the relationship between women knowing the fetal gender and their newborns' birthweight. Four-hundred and eighty-four post-partum mothers before discharge from the maternity unit participated in this study. We collected our data from the women by using two sources: a demographic data questionnaire and reviews of hospital records. For this study, we considered normal birthweight as > 2500 and < 4000 g and low birthweight as < 2500 g. The findings of the study suggested that knowledge of fetal gender was significantly related to the mean birthweight of newborns. When the gender was known prenatally, the mean birthweight of male newborns was significantly higher than that of female newborns. Also knowledge of fetal gender was found to increase the probability of low birthweight among female newborns compared with male newborns. Identifying sociocultural factors that might be linked to the gender preference could improve mothers' and their newborns' well-beings. Nurses have to be aware of the consequences of gender preference such as low birthweight.