Making Equine Ultrasound Safe and Affordable For Breeders

Development of the Preovulatory Follicle

When monitoring follicular growth during estrus, several changes can be noted that would allow the examiner to anticipate ovulation:

  • Follicular size increases progressively and then slows just prior to ovulation.
  • Follicular shape also changes (becomes non-spherical) as ovulation becomes imminent, and may include an obvious ovulation tract developing.
  • Likewise, just prior to ovulation, breakdown of the follicular wall can be detected and seen as a “non-echogenic” area below the follicular wall.

The following image shows progressive development of a preovulatory follicle:


And in the following image, we have what appears to be a large corpus luteum (CL). The mare had a 40 mm follicle two days earlier. It is larger than normal and has cystic appearance, thus will be monitored:




In the videos below, we can see, first, an old CL, and in the second, a new CL …

Old CL …

(Small bright spot located to the left of the larger follicle at about the 6 o’clock position)


New CL …

(At about 24 h, located at about the 7 o’clock position, still non-echogenic center)


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