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Oocyte aspiration, also known as OPU (Ovum Pick-Up), is an advanced reproductive technique used in mares to collect immature eggs (oocytes) directly from the ovaries. These oocytes are then matured and fertilized in the laboratory to produce embryos through a process called ICSI (Intracytoplasmic Sperm Injection).

Why Consider Oocyte Aspiration?

Oocyte aspiration is commonly used in the following situations:

Performance mares still in competition
Older mares with reduced fertility
Mares with uterine problems preventing pregnancy
Mares that repeatedly fail to produce embryos via conventional embryo flushes
Mares that have been injured or are unable to carry a foal

How the Procedure Works:

1. Preparation. The mare is sedated and restrained safely. The procedure is performed using a trans-vaginal ultrasound.

2. Oocyte Collection. A specialized needle is guided through the vaginal wall to the ovary. Follicles are gently aspirated, and the fluid containing the oocytes is collected. The procedure typically takes 20–30 minutes and is minimally invasive.

3. Laboratory Process. The recovered oocytes are sent to a specialized laboratory where they are:

Matured in culture
Fertilized using ICSI (a single sperm is injected into each egg)
Cultured for approximately 7–8 days
The resulting embryos are frozen or transferred into a synchronized recipient mare

Is the Procedure Safe?

Yes. OPU is considered a safe, low-risk procedure when performed by an experienced veterinarian. Most mares resume normal activity within 24 hours.

Complications are rare but may include mild discomfort or temporary ovarian sensitivity.

How Often Can OPU Be Performed?

Oocyte aspiration can typically be performed:

Every 2 weeks in cycling mares
Year-round, including during transitional periods

What Affects Success Rates?

Success depends on several factors:

Mare’s age
Ovarian activity
Semen quality
Laboratory expertise
Number of oocytes recovered

While not every session results in an embryo, multiple sessions significantly increase overall chances of success.

Advantages of Oocyte Aspiration

No need for the mare to carry a pregnancy
Minimal disruption to training or competition
Can be performed outside of the normal breeding season
Useful when traditional embryo transfer is unsuccessful
Maximizes genetic potential of high-value mares

Is Oocyte Aspiration Right for Your Mare?

Each mare is unique. A reproductive evaluation and consultation will help determine whether OPU and ICSI are the best strategy to achieve your breeding goals.

If you would like to discuss whether oocyte aspiration is suitable for your mare, please contact us to arrange a consultation.

When Is the Right Time to Aspirate a Mare?

Unlike conventional embryo transfer, oocyte aspiration (OPU) does not require a dominant pre-ovulatory follicle. In fact, we are usually targeting small to medium-sized follicles, not large ones.

During OPU, we typically aspirate:

Small follicles: ~5–10 mm
Medium follicles: ~10–20 mm

These smaller follicles often contain immature oocytes that can be matured in the laboratory before ICSI.

Large, dominant follicles (30–45 mm) are not the primary target, although they can be aspirated if present.

Interestingly, OPU can be performed:

During estrus
During diestrus
During seasonal transition
Even in early pregnancy (case-dependent)

Because we are not dependent on ovulation, the procedure can be scheduled much more flexibly than traditional breeding.

Does Follicle Size Affect Success?

YES!!

A higher number of small/medium follicles generally increases the number of oocytes recovered
More oocytes = higher overall chance of producing embryos over time
Very small follicles (<5 mm) are less likely to yield usable oocytes
Extremely large, aged follicles may yield lower-quality oocytes

For this reason, many OPU programs are performed at regular 2-week intervals to consistently collect cohorts of developing follicles.

Unlike conventional embryo transfer, OPU–ICSI programs are not dependent on ovulation timing. The objective is recovery of immature cumulus–oocyte complexes (COCs) from antral follicles, followed by in vitro maturation (IVM) and ICSI.

OPU Scheduling Recommendations

Ideally, we would like to see the mares with 10-12 ovarian follicles measuring between 10-20mm in diameter.

Follicles larger than 20mm may also be aspirated; however, the oocyte recovery rate is generally lower.

Mares can be aspirated at any stage of the estrous cycle, but being in early estrus can be beneficial.

Most programs recommend:

OPU every 10–14 days
No requirement for ovulation detection

Ultrasound assessment immediately prior to aspiration to confirm adequate follicular population.