If you’ve ever worried about whether your mare is producing enough hormone to maintain her pregnancy, you’re not alone.

Regumate (Altrenogest) is commonly used to support mares where there is concern that progesterone levels may be too low to sustain a pregnancy.

Sometimes there’s a previous history of pregnancy loss. Sometimes a blood test shows “borderline” progesterone. And sometimes it’s simply a precaution. But how do you know if Regumate is really needed, and for how long?

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A new laboratory test available through the Irish Equine Centre can give vets a much clearer answer.

Why hormone testing matters

In early pregnancy, the mare relies on progesterone from her ovaries to maintain the pregnancy. Later on, the placenta takes over much of that hormone production.

This natural transition is called the luteo-placental shift. In the past, standard progesterone blood tests were used to guide treatment.

However, these tests can sometimes be misleading because they may react with similar hormones and don’t always give a complete picture.

The new test uses highly accurate technology (LC-MS/MS) to measure:

• Progesterone

• 5a-dihydroprogesterone (DHP)

• The ratio between the two

This gives vets a much more precise understanding of what is actually happening in the mare’s body.

What happens during pregnancy?

Here’s how hormone patterns normally look:

Early pregnancy (0–90 days)

• DHP levels should be about half the progesterone level

• If both are low, supplementation with Regumate may be needed

At this stage, the ovaries are still responsible for maintaining pregnancy.

Mid-gestation (120–150 days)

• DHP should now be higher than progesterone. This shows the placenta has taken over hormone production

• If this shift has happened properly, Regumate can usually be stopped (under veterinary guidance)

This is a key turning point. Confirming the shift gives confidence that the mare’s pregnancy is progressing normally and as it should.

Late pregnancy (150 days onwards)

• It is normal for progesterone to fall very low

• DHP becomes the more important hormone to monitor

• DHP should steadily rise toward the last month of pregnancy

• Levels peak just before foaling and drop on the day of birth

Monitoring DHP in late pregnancy can also help in cases where illness, such as placentitis, is a concern.

So what does this mean for you?

This new testing approach helps answer important practical questions such as:

• Does my mare genuinely need Regumate?

• Is it safe to stop supplementation?

• Is the placenta functioning properly?

• Are hormone levels normal for this stage of pregnancy?

It reduces guesswork. It helps avoid unnecessary medication. And most importantly, it supports good welfare decisions for both mare and foal. Ultrasound remains the cornerstone of pregnancy management.

Hormone testing doesn’t replace scanning, it complements it. For breeders, this means more clarity, better timing of treatment decisions, and greater peace of mind throughout pregnancy. If you have concerns about your mare, speak to your vet about whether this newer hormone testing approach could help guide decisions in your case.