This is where parturition (another word for the birthing process) begins. It usually lasts anywhere from 30 minutes to 4 hours, although it can be shorter in mares that are familiar with the routine. During this stage, mares become restless and almost appear colicky. They will look at their flank, swish their tail, and may get up and down frequently. This discomfort during this period is due to uterine contractions and movement of the foal into an appropriate position for foaling. As can be seen in the image below, the foal moves from a dorsopubic position (laying on its back) to a dorso- sacral position (laying on its stomach). It is possible for mares to stop stage one labor if they sense a threat (or if they just want some privacy!), and they can hold off for hours, or even days.
This stage begins with the rupture of the chorioallantois (the main placental membranes) and expulsion of a large amount of allantoic fluid. In other words, the water breaks. It often appears as if the mare has urinated, but the volume is much greater. Occasionally, the sac will not rupture and a "red bag" will result. This is a true emergency and a topic for another time. (If you think you see a red sac protruding from your mare's vulva, call your vet immediately!)
The expulsion of fetal membranes marks the end of parturition. It takes up to 3-4 hours for the placenta to naturally fall out of the mare. It's important not to try to pull it out, as this can cause other problems. If it does not come out in this time period, it is deemed a "retained placenta" and veterinary intervention will be needed. When it does come out, your vet will lay it out in an "F" formation and examine it. Make sure to keep it around until your vet gets there! This can sometimes be an important piece to look at!
Foaling is a beautiful process and anyone involved in the horse world should witness it at least once. While some problems can arise, most foalings occur normally. And no matter how hard you watch, mares always seem to know when you've dozed off for 15 minutes! However and whenever it happens, good luck and enjoy the new addition to the field!