written by: Agata Zagrajczuk (PhD in Animal Reproduction)
Ovulation cannot be predicted only by oestrus symptoms. Accordingly, oestrus does not guarantee ovulation.
The most common technique used to detect ovulation is rectal palpation combined with ultrasound scanning. These two are the ‘gold standard’ for a reproductive veterinarian.
There are a few parameters included in the reproductive clinical assessment:
1. Ovarian structure
Growth of the follicle
Follicle shape and tone
Thickness of follicular wall
2. Uterine structure
Uterine oedema and its changes
Ovarian follicles are spherical structures in the ovaries of every healthy female that contain an oocyte (egg cell). Fertilization and sexual reproduction are not possible without its release.
Follicles are filled with yellowish fluid similar to the blood serum. We can observe them on the scanner as black, round structures. As the oestrus cycle progresses, we observe the follicular growth, recrutation and the selection of the dominant follicle, that achieves the biggest diameter and finally ovulates. Follicle diameter at the time of ovulation usually exceeded 35 mm, with a range of approx. 40-50 mm. Shortly before ovulation the follicle becomes softer, more sensitive under the touch and its regular, round shape changes into a ‘pear-like’ shape. Ovulation is a short event, and it’s visible during the scanning as a follicle collapse.
Sometimes preovulatory follicles don’t ovulate, but that is another story.
Picture 1 - Inactive mare’s ovary
Picture 2 - Preovulatory follicle – diameter 42 mm
Picture 3 - Corpus luteum (CL)– ovary after ovulation
Picture 4 - Preovulatory follicle isolated form mare’s ovary
The uterine oedema is an important indicator of the oestrus cycle, since the endometrium (innermost lining layer of the uterus) must be prepared to receive an embryo. Generally, the uterus during oestrus is heavy and oedematous, with thickened endometrial folds, while during dioestrus (luteal) cycle stage, normal oedema and endometrial folds are not observed. However, an accumulation of fluid inside the uterus and/or exaggerated oedema are usually indicators of uterine inflammation and thus need to be diagnosed by a vet.
Additional examinations, that may help in the cycle evaluation of the mare and at the same time exclude some breeding problems, like pooling, trauma of the reproductive tract and persistent hymen, is the speculum/vaginal palpation exam.
During this exam, the appearance of the cervix depends on the cycle stage. During oestrus the cervix is relaxed and oedematous, lowered onto the vaginal floor in the frontal part of the vagina. The cervix is dilated, and the vet may easily palpate it. The mucus will be pink and moist.
During the luteal (dioestrus) stage or pregnancy, you will see the opposite, with the cervix tightly closed, pale and dry/covered with sticky mucus. During the seasonal anoestrus we can observe the same symptoms.