This is a very important question because it can have a lot of repercussions. If a mare goes far past her due date, her health can be at risk, as will the foal. A mare that goes past her due date may have reduced blood flow to her uterus, which can cause the placenta to rupture. They can also have complications with their foals, such as difficult births, retained placenta, and fetal distress. Most foals born after an extended gestation are small in size due to delayed uterine development. However, most mares will gestate a bit longer than 11 months although even gestation periods of 380 days are still considered normal. The longest successful gestation recorded so far was 445 days.