In the department of “you just never know,” this story came to me from a horse-loving acquaintance. He knew a man with a pair of Quarter Horse mares who bred them to a nice stallion. The mares had cycled close together, and sure enough nearly a year later they had grown heavy, were put under lights, began dripping milk, and then one evening the first mare went into labor.
About half an hour later, the second mare went into labor. This was too much for the owner, who called the vet to come assist.
The vet arrived in time to see the first mare comfortably lying in the hay with a newborn. Hers was a simple delivery, but the second mare appeared to be having difficulty.
She lay in the straw, straining and sweating, but without any sign of a foal. The vet judged this had gone on long enough, donned an exam glove, and went in search of the foal.
There wasn’t one.
The vet couldn’t believe it, took his ultrasound probe and did a 360 degree sweep. The mare was not in foal.
Apparently, she had a sympathetic pregnancy, which allowed her to gain weight, lactate, and even go into labor, but she did not have the final consideration: a foal.