Udder enlargement is the result of hormonal signaling from the maturing fetus and placenta, toward the end of gestation. If this signaling occurs too soon, then udder enlargement will be premature. In most cases, udder enlargement begins about 4-6 weeks prior to the predicted foaling date, or at about 280 days.

Different mares have different patterns of udder development, but generally, the udder begins to enlarge at about 4-6 weeks prior to the foaling date. In most mares, the udder gradually enlarges up until about 2 weeks prior to birth. Usually there is a significant increase in size of the udder then, followed by a final increase right before foaling.

In the final days before foaling, the teats become tight and swollen with fluid. At no time should there be significant dripping of milk from the teats. Premature lactation is a potential emergency because it indicates impending foaling. For normal immunity, the foal will need every bit of colostrum (first milk) that the mare has produced.

If udder development occurs too early (before 270 days gestation) or this is accompanied by dripping of milk it may be an indication of a problem with the fetus or placenta and impending foaling. So this is an emergency.