Maximum strain of parturient heart occurs during:

“Significant hemodynamic alterations are apparent early in pregnancy, women with severe cardiac dysfunction may experience worsening of heart failure before mid pregnancy. In others, heart failure develops after 28 weeks, when pregnancy induced hypervolemia is maximal (32 weeks). In the majority, however heart failure develops peripartum when the physiological capability for rapid changes in cardiac out put may be overwhelmed in presence of structural cardiac disease.” Williams 22/e, p 1018, 23/e, p 958, 959 Reading the above text, from Williams Obs., it is clear that maximum chances of heart failure are in the peripartum period. But it is not clear whether maximum chances are during labour or immediate postpartum. Dutta Obs. 7/e, p53 provides answer to this: “The cardiac output starts to increase from 5th week of pregnancy, reaches its peak 40-50% at about 30-34 weeks. Thereafter the cardiac output remains static till term”. “Cardiac output increases further during labour (+50%) and immediately following delivery (+70%) over the pre labour values.” So, maximum chances of heart failure are in immediate postpartum period when cardiac output is maximum.