Editing Horse breeding (section)

=== Effects on the reproductive system during the estrous cycle ===
Changes in hormone levels can have great effects on the physical characteristics of the reproductive organs of the mare, thereby preparing, or preventing, her from conceiving.
* '''Uterus''': increased levels of estrogen during estrus cause [[edema]] within the uterus, making it feel heavier, and the uterus loses its tone. This edema decreases following ovulation, and the muscular tone increases. High levels of progesterone do not cause edema within the uterus. The uterus becomes flaccid during anestrus.
* '''Cervix''': the cervix starts to relax right before estrus occurs, with maximal relaxation around the time of ovulation. The secretions of the cervix increase. High progesterone levels (during diestrus) cause the cervix to close and become toned.
* '''Vagina''': the portion of the vagina near the cervix becomes engorged with blood right before estrus. The vagina becomes relaxed and secretions increase.<ref>{{Cite web|url=https://thehorse.com/127989/basics-of-life/|title=Basics of Life|date=2006-12-01|website=The Horse|language=en-US|access-date=2019-06-11}}</ref>
* '''Vulva''': relaxes right before estrus begins.  Becomes dry, and closes more tightly, during diestrus.<ref name="Samper2009">{{cite book|author=Juan C. Samper|title=Equine Breeding Management and Artificial Insemination|url=https://books.google.com/books?id=14-EHWvwIoEC&q=estrus+OR+diestrus|date=1 January 2009|publisher=Elsevier Health Sciences|isbn=978-1-4160-5234-0}}</ref>