In horses, the time from conception (When a mare becomes pregnant) to the time the foal is born is approximately 340 days on average. It is possible for the mare to deliver 20-30 days in one direction or the other.
The first three and a half months of gestation, the mare shows little sign of pregnancy. An equine vet is able to test for pregnancy anytime after the first 30 days.
After the first five months, the foal really begins to develop. s/he begins to grow eyelids and eyelashes!
After seven and a half months, the mare starts to be noticeably larger in the abdomen, and the foal begins to rapidly gain weight. A healthy foal will gain around a pound a day.
Around ten months of gestation, the mare's udders will begin to produce milk. Her teats will become distended. This is called "bagging up".