I had complete placenta previa and went for my c-section at 36 weeks because I started spotting, so my gynae decided not to wait any longer rather than risk more bleeding. From what I heard from other mums though, timing really depends on how severe yours is and whether there's any bleeding, so it's quite individual lah. Best to have a detailed chat with your gynae about your specific situation and what they're monitoring for, since they'll have the full picture from your scans.
Anonymous