Abstract: Objective To investigate the effect of heat exposure during the second week of pregnancy on placental development and intrauterine growth of fetal rats. Methods Twenty-four pregnant rats were either exposed or not to a temperature of 35±1 ° C during the second week of pregnancy. The body weight gain of the pregnant rats was measured regularly, and in late pregnancy, the pregnant rats were dissected and the number, weight, length, tail length, appearance of the offspring rats, number of live and still births, and the placental weight were recorded. The expressions of HSP70, Bax and Bcl-2 in the placenta were determined. Results Compared with the control group, the pregnant rats in heat exposure group had significantly lower body weight at the end of pregnancy and gestational weight gain, and the body weight, body length and tail length of the offspring rats were also significantly lower or smaller (P<0.05). The placental weight was comparable between the two groups. The placental expressions of HSP70, Bax, and Bcl-2 were significantly higher in the heat exposure group than in the control group (P<0.05). Conclusion Heat exposure during the second trimester of pregnancy has adverse effects on placental development and intrauterine growth of the fetal rats by inducing heat shock response of placental tissue and apoptosis of the placental cells.