According to the recent 2015 Nutrition Survey, the prevalence of being underweight (Body Mass Index, BMI <18.5 kg/m²) among women in their 20s is 22.3%. Women of childbearing age tend to have a lower intake of protein and their total energy intake is lower than the requirements established by the 2015 Dietary Reference Intakes for Japanese. There is a growing body of evidence showing that underweight women tend to bear small babies and that these babies have an increased risk of diabetes or cancer in their adulthood. In order to prevent Japanese women from bearing small babies, the literature has suggested that women of childbearing age should be encouraged to remain at a normal weight before pregnancy. For optimal weight gain during pregnancy, existing guidelines recommend different ranges of weight gain based on prepregnancy BMI. Owing to the absence of official GWG recommendations in Asian countries, including China and Taiwan, the US Institute of Medicine (IOM) guidelines are generally followed. However, Asian women are smaller and experience lower weight gains; therefore, excessive weight gain may lead to harmful events including macrosomia, preterm birth, preeclampsia, gestational diabetes, pregnancy-induced hypertension, and short- and long-term postpartum weight retention. Thus, an accurate GWG range should be determined for Asian women. We introduce one epidemiological study in which the optimal weight gain range was investigated by analyzing receiver-operating characteristic curves together with potential research ideas in this field with the aim of encouraging young researchers to solve this public health problem affecting mothers and children.