Background: This is the first study to evaluate changes in postpartum pulmonary circulation in a novel pregnant rat model of monocrotaline (MCT)-induced pulmonary arterial hypertension (PAH). Methods: Female rats were randomly divided into two groups: the MCT-treated pregnant group, in which rats were injected with MCT (40 mg/kg) at the age of 7 weeks, and the pregnant group, in which rats were injected with the same volume of 0.9% saline at the same age. Rats from both groups were mated at the age of 9 weeks. General condition information, hemodynamic data and pulmonary tissues were collected from pregnant rats from the two groups on the 18th day after successful mating (T1) and the 1st (T2), 3rd (T3), and 7th days after delivery (T4). Results: The MCT-treated pregnant group exhibited a greater mean pulmonary artery pressure (mPAP) (P<0.01) and Fulton's Index (P<0.01) than the pregnant group at each time point. Lung tissues from the MCT-treated pregnant group showed pulmonary vascular hyperplasia and occlusive changes. The mPAP and the occluded pulmonary artery density in the MCT-treated pregnant group increased after delivery (P<0.01) and significantly increased at T3 compared with T2 (P<0.05) but was not further increased at T4 (P>0.05). Conclusions: Pregnant rats with PAH exhibited increased mPAP after delivery accompanied by a significant increase in the occluded pulmonary artery density, which may have contributed to the increased mortality rate of pregnant rats with PAH after delivery.