The minimal number of normal, motile sperm cells deposited in the uterus that are needed to achieve fertilization has been estimated at 100 to 500 million cells. The lower figure probably is about right for artificial insemination, and the higher number for natural service. It is interesting to note that the many billions of sperm that do not migrate into the oviduct are not totally wasted. For the majority of stallions with adequate sperm numbers, the limiting factor affecting the number of mares they can breed is their libido, or their willingness to breed mares. This is another stallion factor that is affected by season.
Long days equal more sexual desire and more sperm cells produced.
So, we have delivered a fertile ovum into the oviduct of a mare which has been covered by a fertile stallion, so that healthy sperm are available. Cell division commences immediately after the fertilization process, and the embryo begins its descent through the oviduct to the uterus. For the next week, as the embryo continues to develop, it migrates through the uterus, contacting the lining as a signal to the mare that she is pregnant. By about 15-16 days, the migrating stops and the embryo is fixed in one place, usually at the base of one uterine horn. If the signal has been received and interpreted, the uterus will not release prostaglandin, so that progesterone production will be continued by the CL, and the mare will not return to estrus.