A barren mare is a mare that has been bred before, but did not carry a foal this year. The primary cause of open mares is uterine scarring from poor postpartum care following her earlier pregnancies. The scarring is most commonly caused by genital reproductive infections. It is defined as the period of fetal development and growth to a state at which the new individual is sufficiently mature to survive outside the uterus. An ovum may be fertilized up to four days after ovulation.
Fertilization occurs in the oviduct leading to the uterus.
The ovum travels down the oviduct and arrives in the uterus about five days after conception. In the mare, the fertilized ovum, or embryo, will continue to float unattached in the uterus for a comparatively long period. Implantation in the uterine lining starts at day 45 and is not complete until around 110 days. During this time, primary organ development and tissue differentiation takes place, but maintenance of the pregnancy is precarious. At 120 days, the developing fetus has hooves, external genitalia, and ergots.