Could hormones produced during pregnancy cause my pregnant mare to lose all of her winter hair?

Yes, this could be due to hormones of pregnancy. The hormone prolactin is involved with hair shedding in horses, and is involved in growth and development of the mammary gland in late gestation prior to foaling. The rise in prolactin prior to foaling is normal and should not be cause for alarm. This may be the cause for the hair loss that you report. If you have further concerns, you may want to have your veterinarian take a blood sample and submit it to a laboratory for a hormone profile analysis. This can tell you if any of the hormones are “out of whack”.