Past studies have shown that cerebral autoregulation is a physiologic process that maintains blood flow at an appropriate level. Impairment of cerebral autoregulation during pregnancy may contribute to further diseases like preeclampsia and eclampsia. However, little is known about the normative cerebrovascular adaptation during pregnancy. Two photon microscopy has become an important tool for living brain function research due to its deeper detection, higher resolution, lower photodamage compared to wide-field and confocal microscopy. In the present study we observed adaptive changes in the blood flow and vessel diameter from prepregnancy to late pregnancy by in vivo two-photon imaging. 12 female C57 mice were randomly divided into four groups: non-pregnancy, early-pregnancy, middlepregnancy, late pregnancy. A thinned skull window was made over the motor cortex. One day later, Rhodamin B was injected intraperitoneally to label the vessels 15 minutes before beginning the imaging experiments. The animal was anesthetized and placed under two photon microscope to observe; blood flow signal pictures were taken using line-scan and analysed by blood flow analysis software programmed. The blood flow velocity of arteriovenous vessels and capillaries increased significantly in middle and late pregnancy compared to non-pregnancy and early-pregnancy; vascular diameters in middle and late pregnancy also became bigger than that in non-pregnancy and early-pregnancy. Both the blood flow velocity and diameter of cortical micro vessels increased significantly from nonpregnancy and early-pregnancy to middle and late pregnancy. Such results demonstrated a progressive cerebrovascular adaptation from prepregnancy to late pregnancy.