Ova (singular: ovum) are the female reproductive cells produced by the mare’s ovaries. Each ovum is a large, spherical cell containing the haploid genetic complement of the dam; when fertilised by a spermatozoon it forms the zygote from which the foal develops.
The mare is seasonally polyoestrous: under long-day photoperiod she cycles at roughly 21-day intervals, each cycle culminating in ovulation (the release of a single ovum from a dominant follicle on one ovary). Twin ovulations occur in roughly 15 to 25 percent of cycles and are actively managed in breeding programmes because twin pregnancies in horses carry high risk of abortion and foal loss. Ova are not stored; each is produced de novo within its follicle over the 7 to 10 days preceding ovulation and is viable for fertilisation for only a few hours after release.
At fertilisation the ovum completes its second meiotic division, expelling a polar body and restoring the diploid chromosome number (64 in the domestic horse). The fertilised ovum descends through the oviduct into the uterus and undergoes several days of free mobility before fixation at approximately day 16 of gestation.