Fescue hay harbors a symbiotic endophyte called Neotyphodium coenophialum. This means that both parties benefit from the relationship: the endophtye enjoys nutrition from the plant and the plant achieves chemical protection against grazers via an ergopeptine alkaloid called ergovaline. Consumption of these alkaloid chemicals leads to a condition called fescue toxicity in horses. As far as I am aware, no studies have shown the effect of ergot alkaloids in horses other than with late-gestation mares.

In pregnant mares that have been exposed to fescue poisoning, the foal is often carried for 27 to 40 days longer than normal if it has not been aborted. Given that your horse has been pregnant for approximately 38 days longer than should be expected, it seems likely that your horse has been affected. During the prolonged gestation the foals continue to grow and the birth is often difficult because of the size of the foal. In addition, affected mares may not produce enough milk or milk of sufficient quality. I therefore recommend that you notify your vet of your concerns and be very prepared for a difficult birth and attentive after-care.