Dose–response relationships between maternal urinary cotinine and placental weight and ratio of placental weight to birth weight: The Japan Environment and Children's Study

•We studied response of placental weight and its ratio to birth weight to dose of maternal urinary cotinine.•Dose–response between cotinine concentration and placental weight formed an inverted Ushape.•The dose–response between cotinine concentration and placental weight differed for male and female offspring.•With low cotinine, the placenta enlarges to compensate for hypoxia; at high cotinine, hypoxia may cause Placental cell death.