The effect of maternal weight gain during pregnancy on fetal, infant, and childhood death and on cognitive development was analyzed by examining the conception products of 2590 women who participated in the Child Health and Development Studies of the School of Public Health, University of California, Berkeley. Although there was a strong inverse association between maternal weight gain and fetal and neonatal death, the implications of this are uncertain and are herein discussed. When the analysis was confined to pregnancies with gestational periods greater than 35 weeks, there was no effect of maternal weight gain on fetal, neonatal, infant, or childhood death. Children whose mothers gained between 5 and 29 1b during their pregnancy scored higher on the Raven Coloured Progressive Matrices test at age 5 than did those whose mothers gained less than 5 or more than 29 lb. Within the 5- to 29-lb group there were no significant differences in test scores among the different categories of maternal weight gain. (Obstct Gynecol 60:689, 1982)