The aim of this study was to identify the effect of hyperglycaemia on placentas of gestational diabetes mellitus (GDM) women with macrosomia and normal pre-pregnancy body mass index (BMI), and uncover the molecular mechanism of hyperglycaemia on trophoblast cells . GDM women with normal pre-pregnancy BMI were divided into GM group (macrosomia,  30) and GN group (normal birth weight,  35). The study showed GM group had more adverse pregnancy outcomes and higher levels of gestational weight gain, blood glucose and triglyceride. After adjustment for confounding factors, just the fasting plasma glucose level and HbA1c percentage were related to the incidence of GDM-induced macrosomia with normal pre-pregnancy BMI. Meanwhile, the fasting blood glucose was closely related to the placental weight and placental PCNA expression. Furthermore, the model for placenta showed that hyperglycaemia significantly promoted trophoblast cell proliferation and activated ERK1/2 phosphorylation. ERK1/2 inhibitor markedly suppressed hyperglycaemia-induced trophoblastic proliferation. The fasting plasma glucose and placenta are closely related with the development of GDM-induced macrosomia with normal pre-pregnancy BMI. The mechanism may be hyperglycaemia promotes trophoblast cell proliferation via ERK1/2 signalling. It provides scientific evidence for optimising outcomes of GDM women with normal pre-pregnancy BMI.IMPACT STATEMENT Gestational diabetes mellitus (GDM) is one of the strongest risk factors correlated with macrosomia. The hyperglycaemic intrauterine environment affects not only the foetus but also the placental development and function in humans and experimental rodents. However, placental abnormalities associated with maternal diabetes have been inconsistently reported, possibly because of population differences in pre-pregnancy weight, diabetes types, glycemic control or pregnancy complication, and the molecular mechanism of hyperglycaemia on trophoblast cells was not clearly stated. This is the first study to identify the effect of hyperglycaemia on placentas of gestational diabetes mellitus (GDM) women with macrosomia and normal pre-pregnancy body mass index (BMI), and uncover the molecular mechanism of hyperglycaemia on trophoblast cells . Understanding placental changes in the environment of abnormal glucose metabolism which can establish the maternal-placental-foetal interface dysfunction as a potential source of adverse pregnancy outcomes is very necessary. Our study found the fasting plasma glucose and placenta are closely related with the development of GDM-induced macrosomia with normal pre-pregnancy BMI. The mechanism may be hyperglycaemia promotes trophoblast cell proliferation via ERK1/2 signalling. It provides scientific evidence for optimising outcomes of GDM women with normal pre-pregnancy BMI, and could be used for the following studies of relationship between placenta and childhood complications.