In rare cases, a mare may exhibit symptoms of pregnancy but not actually be pregnant. This condition is known as false pregnancy or pseudopregnancy. The mare may display signs such as weight gain, enlarged mammary glands, and even behavioral changes, but there is no foal developing inside her uterus.
Another possibility is that the mare is experiencing a molar pregnancy, wherein abnormal cells develop in the uterus instead of a foal. This can lead to the formation of a mass or tumor that may need to be removed surgically.
One possible explanation for a horse being pregnant but not giving birth to a foal is hybridization. Horses can crossbreed with other equine species, such as donkeys or zebras, which can result in a hybrid fetus. However, these hybrids are often sterile and cannot reproduce.
Another possible cause is the development of a teratoma, which is a type of tumor that can contain tissues from various organs, including the ovaries. In rare cases, a teratoma can develop into a fetus-like structure, although it is not a viable or living organism.
Embryonic diapause, also known as delayed implantation, is a phenomenon that occurs in some animals, including horses. This happens when a fertilized egg enters a state of suspended development and does not implant in the uterus immediately. In some cases, the embryo may remain in a state of diapause for several months before resuming development and implanting in the uterus.
A parasitic twin, also known as an asymmetrical conjoined twin, occurs when one twin fails to fully develop and becomes attached to the body of the other twin. This can result in a malformed fetus that may not be viable or may require surgical removal.