[Objective] The present criteria for maternal weight gain during pregnancy proposed by Japan Society for Obstetrics and Gynecology were reevaluated by investigating the changes in maternal and neonatal birth weights that occurred during the period of 15 years beginning from 1988.<br> [Materials and Methods] The subjects included 3310 pregnant women who were managed in Kaibara Prefectural Hospital between January 1988 and December 2002. These subjects were classified into the following three categories based on their pre-pregnancy constitutions: underweight (Body Mass Index [BMI] < 18), normal-weight (18 ± BMI ± 24), and overweight (BMI > 24). In addition, the incidence rates of low-birth-weight infants (<2.5kg), macrosomic infants, light-for-date (LFD), heavy-for-date (HFD) infants, and pregnancy-induced hypertension were investigated. All the subjects in each category were further classified into three groups based on the total weight gain during pregnancy: excess-, optimum-, and insufficient-weight-gain groups. Furthermore, the annual changes in the distribution of pregnant women among these three groups were also investigated.<br> [Results] In 1988, the maternal weight gain in all categories was 12.0 ± 3.7kg (mean ± SD) ; it significantly decreased to 10.0 ± 3.9kg in 2002. Neonatal birth weight also decreased from 3114 ± 414g in 1988 to 3040 ± 384g in 2002. These changes were distinct in the overweight category; in addition, the incidence of low-birth-weight infants also increased. In the underweight category, neonatal birth weight decreased in the <10-kg-weight-gain group, and incidence of HFD as well as pregnancy-induced hypertension increased in the >14-kg-weight-gain group. In the normal-weight category, incidence of low-birth-weight infants significantly increased in the <7-kg-weight-gain group and that of HFD and macrosomic infants increased in the >13-kg-weight-gain group. Pregnancy-induced hypertension also increased according to the increase in maternal body weight. In the overweight category, incidence of HFD and macrosomic infants significantly increased in the >7-kg-weight-gain group. Based on this data, the following new criteria for weight gain were defined: 10-14kg in the underweight category, 7-13kg in normal-weight category, and <7kg in the overweight category. On the other hand, a few subjects with optimum weight gain were present in all categories of pre-pregnancy constitutions. Subjects with insufficient weight gain were conspicuous in the underweight category and those with excess weight gain in both normal-weight and overweight categories. <br> [Discussion] During the period of 15 years beginning from 1988, both maternal weight gain and neonatal birth weight decreased. Based on the present criteria for maternal weight gain, suppression in maternal weight gain during pregnancy should be strictly controlled. The present criteria consider gestational edema as a determinant of optimum maternal weight gain. Consequently, it is considered that the upper limit of the optimum maternal weight gain in the normal-weight category is being lowered. However in future, new criteria for optimum maternal weight gain corresponding to the change in the classification of pregnancy-induced hypertension-the so-called toxemia of pregnancy-should be widely discussed in Japan. [Adv Obstet Gynecol, 57(2) : 121-130, 2005(H17.5)]<br>