Understanding Estrus on Ultrasound


If you caught last week\’s post on My Stable Life, I began a series about ultrasounding mares. I find the procedure fascinating and I\’m always intrigued to learn more when our mares go into the vet clinic for breeding season. If you\’d like to see that first post, click here. Today, I\’m going to help give you a better understanding of estrus and diestrus. Or in other words – what it means when a mare is in heat, or not. I will explain what hormones are cycling through the mare\’s system in each phase and offer a description of the image that appears on ultrasound imaging as a result.




What is happening: Diestrus. The mare is not in heat. Diestrus means the mare will reject the advances of a stallion during this 14-16 day period in the estrous cycle.
What it means: High progesterone – luteal phase.
Image on Ultrasound: A homogenous dense tissue is seen while scanning uterine horns and body. The uterus is toned. Often, a follicle may be seen in the ovary however, it is inactive if there is no presence of edema in the uterus and therefore, progesterone. It is also possible for several follicles to be observed on the ovary – as the mare approaches estrus (heat), one follicle will become dominant and the others will regress.




What is happening: Estrus. The mare is in heat and there is a presence of uterine edema. Estrus means the mare will accept the advances of a stallion during this period of follicular growth.
What it means: Low progesterone and high estrogen.
Image on Ultrasound: Visible as a “sand dollar” or “orange slice” image in the uterus. This image occurs because of the characteristics of the uterusʼ changes at this time. The uterus has several endometrial folds that increase the surface area of the endometrium. During estrus, these folds become filled with fluid, giving the area the appearance of an orange slice.

Sorry, no photo 🙁

What is happening: Follicular presence on the ovary that is increasing in size over the estrus period.
What it means: An active follicle is producing estrogen and causes edema in the uterus. The mare is approaching ovulation.
Image on Ultrasound: The best prediction of when ovulation will occur is follicular size and luckily, the preovulatory follicle in the horse is the largest of all domestic species (40-50 mm). Since the ovarian follicle is also filled with a clear non-echogenic fluid, this makes a follicle very easy to see with ultrasonography. The follicular wall may increase in thickness or the follicle may also change from a spherical shape, to more of a
triangular or “tear drop” shape as the mare gets closer to ovulating. It can also feel softer, rather than hard and toned, as ovulation becomes imminent.

Sorry, no photo 🙁

What is happening: The ruptured/ovulated follicle on the ovary develops into a corpus hemorrahagicum (CH) or a corpus luteum (CL).
What it means: The mare has ovulated.
Image on Ultrasound: A CH looks similar to a follicle on an ultrasound however, there will be specs or a hemorrhage of blood in this follicle. It has a black center (because it has clear fluid running through it) and the specs of blood will look like spider webs or have a “lacy” appearance running through the center (as blood reflects the ultrasound waves).

Sorry, no photo 🙁
What is happening: A corpus luteum (CL) develops on an ovary.
What it means: The mare has ovulated. A CL produces progesterone, which holds a pregnancy until day 37 of gestation. After this time, the endometrial cups on the conceptus cause the mareʼs system to recognize the pregnancy.
Image on Ultrasound: A dense, bright white structure appears on the ovary. The center of the corpus luteum is completely filled in, making it highly echogenic. 50 per cent of all ovulations result in a corpus hemorrahagicum and the other 50 per cent result in a corpus luteum. Both mean the mare has ovulated and there is no difference in pregnancy rate between ovulations resulting in CH or CL.
* If the mare is not pregnant, the cycle starts over and the follicular cycle repeats itself.

When My Stable Life returns, we\’ll observe the early stages in a typical equine pregnancy and follow along with a follicle.



1 thought on “Understanding Estrus on Ultrasound”

  1. Pingback: Rectal Palpations and Getting Your Mare Pregnant! | Equine Reproduction

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