🍼 Progesterone Supplementation in Mares: When It’s Needed (and When It’s Not) – 2025 Vet Insight 🐴

By Dr Duncan Houston BVSc

It's common for owners and breeders to use progesterone supplements in pregnant mares, believing it helps prevent abortion. But in 2025, we now know many of these cases likely don’t need supplementation—especially beyond day 100 of pregnancy. Let’s break down what’s necessary and what’s myth. 💊📉

📅 Pregnancy Timeline & Progesterone Production

• 🗓️ First 100 days: Progesterone is produced by the ovary.
• 🗓️ After day 100: The placenta produces sufficient progesterone.

Unless there’s a specific veterinary reason, supplementing with products like Regu-Mate® or injectable progesterone after day 100 is usually unnecessary.

💵 The Cost of Unneeded Hormones

Administering Regu-Mate can cost up to $3/day—that’s nearly $270 for a 3-month period. Without a compelling medical reason, this becomes an avoidable expense, especially when science suggests most mares do fine without it.

🔬 When Is Supplementation Actually Necessary?

Dr. Peter Daels and other equine reproductive experts indicate that supplementation is justified only if:

• A blood test in early pregnancy shows low progesterone.
• The mare has undergone ovary removal.
• The mare becomes acutely ill during late pregnancy.

Important note: After day 100, blood tests are no longer reliable for assessing progesterone, since placenta-produced hormones aren’t detectable in the same way.

⚠️ Hidden Risk of Routine Regu-Mate Use

If your mare aborts while on Regu-Mate, she may not show any signs of coming back into heat—leading to false assumptions that she’s still pregnant. 😟

This is why regular pregnancy checks are essential when using progesterone. Don’t assume everything’s fine just because the supplementation continues.

✅ The Only Proven Products

There are many progesterone-based products on the market, but the only ones proven effective in mares are:

• Regu-Mate® (oral)
• Progesterone injections

🩺 When in Doubt, Ask Your Vet

Before starting or continuing any hormone therapy, work closely with your equine veterinarian to assess need. The reproductive health of your mare—and your wallet—depends on it!

Download the Ask A Vet app or visit AskAVet.com to consult with a licensed vet about proper mare care, progesterone use, or any equine reproductive concerns. 🐴📱

— Dr Duncan Houston BVSc