Equine Breeding Season
It’s almost that time of year again — equine breeding season. Are you prepared?
Equine breeding season is right around the corner. Now is the time to ensure your equipment, protocols, and team are ready for a smooth and successful season. To help you prepare, here are a few practical ultrasound tips and reminders:
Transrectal Ultrasound:
Depending on the indication and stage of the reproductive cycle, examinations may be performed transrectally or transabdominally. For most transrectal reproductive exams, a linear probe is preferred. Linear probes provide:
- Minimal distortion of reproductive structures
- Greater accuracy for measurement and follicle scoring
- Consistent image quality for pregnancy diagnosis
While a curvilinear rectal probe can provide acceptable image quality if needed, a linear rectal probe will provide a much better image quality. A linear rectal probe is a higher frequency shallower image which works great when scanning a mare’s reproductive tract. For clinicians performing a high volume of equine reproductive work, the improved accuracy of a linear probe outweighs the deeper wider field of view offered by a curvilinear probe.
Important reminder:
Prior to any transrectal sonographic examination, thorough evacuation of fecal material from the rectum is essential for optimal image quality and patient safety.
Transabdominal Ultrasound:
For transabdominal exams in mares, a curvilinear rectal probe or a curvilinear T-handle probe are both appropriate choices. These probes are lower frequency and therefore offer a deeper field of view. In most cases, clipping is unnecessary unless the mare has a particularly heavy winter coat. Alcohol alone is typically sufficient to achieve adequate probe contact.
Stages of Equine Gestation
| Gestational Age (Day Post-Ovulation) | Developmental Event | Anatomical / Physiological Description |
|---|---|---|
| Day 0 | Ovulation | Release of the oocyte from the dominant follicle; formation of the corpus luteum (CL). |
| Day 5-6 | Uterine Entry | The early blastocyst (conceptus) enters the uterine lumen from the oviduct. |
| Day 6-16 | Intrauterine Migration | The spherical conceptus (encased in a glycoprotein capsule) undergoes continuous mobility throughout both uterine horns and uterine body. This mobility is mediated by prostaglandin-induced myometrial contractions and is essential for maternal recognition of pregnancy (MRP) to prevent luteolysis. |
| Day 16-17 | Fixation | The conceptus becomes spatially fixed at the base of one uterine horn due to increasing diameter and reduced uterine tone. Fixation is not equivalent to implantation; the conceptus remains unattached at the cellular level. |
| Day 20-30 | Early Apposition | The trophoblast begins closer apposition to the endometrial epithelium; the glycoprotein capsule gradually thins. No true interdigitation yet occurs. |
| Day 35-40 | Initial Attachment (Implantation) | The trophoblast establishes stable microvillar interdigitation with the endometrial luminal epithelium. This marks the beginning of definitive chorioallantoic placentation. |
| Day 40 | Progressive Placentation | Development of a diffuse, epitheliochorial placenta with extensive microcotyledonary interdigitation across the endometrium. |
Based upon the chart above you can decide when the best time is to ultrasound the mares.
Ideal Times for Ultrasounding
Ideally, for confirmation of pregnancy, an exam should be performed between 13 and 16 days after breeding. At this point, the embryo attaches at the base of a uterine horn and will appear as a circular hypoechoic structure with a hyperechoic rim. Pregnancy detection accuracy approaches ~99%, and twin vesicles can be identified before fixation.
The next crucial exam is at six months to observe fetal viability and placental health; however, many owners may request an exam to sex a foal. Fetal sexing is best done transrectally at 45-60 days of gestation and is very similar in technique to sexing bovine fetuses. It can be done after day 90 transabdominally, but obtaining an adequate view may be challenging. The six-month exam should confirm a heartbeat and identify any major physical deformities.
