From research done at the University of Maryland, we found that age of the mare at foaling, sex of the foal, and time of year bred contributed to the length of the pregnancy.

Statistical analysis showed that the calculation of length of gestation related to these factors was a much more accurate estimate of the normally used 340 day average.

Later research showed that the best estimate of a specific mare’s length of gestation was determined from prior pregnancies. Without this kind of individual mare data, this chart is usually a better estimate for the length of gestation than the 340 day average.

How to Use the Chart: Locate the age of the mare at time of foaling along the top of the chart, then locate the date she was last breed along the side. Follow the column down and the row across till they intersect. This is the predicted foaling date. For example, the line is highlighted in the chart what predicts the foaling date for a mare that will be 15 years old when she foals and that was bred on March 2nd. It predicts that she will foal about February 17th.

There is a 6 day foaling window around this date. On the average colts are 3 days early and fillies are 3 days late.

Remember that the best estimate of the length of a mare's pregnancy comes from the record of prior pregnancies. However when that information isn't available, this Chart may prove a valuable tool.