Neurodevelopmental disorders (NDDs) in children are associated with a complex combination of genetic and/or environmental factors. Pre-/perinatal events are major known environmental suboptimal factors, and their individual and combined contributions vary. This study investigated the association between pre-/perinatal reduced optimality and child development observed by parents at 1 month, as well as NDDs at 3 years of age (i.e., motor delay, intellectual disability, developmental language disorder, and autism spectrum disorder), in the context of the Japan Environment and Children’s Study. The study also assessed whether child development at 1 month predicted NDDs at 3 years of age. Associations between 25 pre-/perinatal factors and (a) developmental concerns at 1 month of age and (b) NDDs at 3 years were analyzed (n = 71,682). Binomial regression models were used to investigate risk ratios of the developmental outcome at each time point for total pre-/perinatal reduced optimality scale scores, as well as for individual pre-/perinatal factors of the reduced optimality scale. Finally, we assessed the ability of parental observations of offspring development at 1 month to predict NDDs at 3 years. Total reduced optimality scores were positively associated with 1-month developmental concerns and 3-year NDDs, with higher scores (i.e., a reduction in optimality) associated with an increased risk of both NDDs and earlier parental concerns. Neonatal transportation, epidural analgesia, advanced maternal age, cesarean section delivery, Apgar score ≤8, and hyperbilirubinemia were identified as individual risk factors for 3-year NDDs, overlapping with 14 risk factors for 1-month developmental concerns except Apgar score ≤8. Among six developmental items assessed at 1 month of age, concerns about gross motor function and difficulty holding/trouble calming down had the strongest associations with later-diagnosed motor delay and autism spectrum disorder, respectively. Five perinatal factors and advanced maternal age were associated with NDD at 3 years of age, as were early parental developmental concerns regarding their offspring’s overall development, indicating the importance of careful follow-up of offspring born with pre-/perinatal reduced optimality. The results also implicated early parental concerns, as early as 1 month, may also be a useful indicator of later NDD status.