Progesterone Supplementation in Mares

Progesterone Supplementation in Mares: When It’s Needed and When It’s Not

By Dr Duncan Houston

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Quick Answer

Most pregnant mares do not need progesterone supplementation, especially after day 100 of pregnancy when the placenta produces sufficient hormones. Supplementation is only indicated in specific cases such as confirmed low progesterone early in pregnancy or certain medical conditions. Routine use without diagnosis is often unnecessary.

Progesterone in mares is one of those topics where tradition has stuck around longer than the evidence.

A lot of mares are supplemented “just in case”. The intention is good. Prevent pregnancy loss, reduce risk, cover all bases.

But in many cases, it is simply not needed.

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What to do before using progesterone

Before starting supplementation:

1. Confirm pregnancy properly

2. Assess risk factors

3. Run blood tests early if indicated

4. Discuss with your vet before starting

Using progesterone without a clear reason is where most problems begin.

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How progesterone works in pregnancy

Progesterone is essential for maintaining pregnancy.

In mares:

• Early pregnancy (up to ~day 100)
  Progesterone is produced by the ovaries

• After ~day 100
  The placenta takes over hormone production

Once the placenta is producing adequate hormones, supplementation is usually unnecessary.

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When progesterone supplementation is actually needed

There are specific situations where it is justified:

• Low progesterone confirmed on blood testing early in pregnancy

• Loss or dysfunction of ovarian tissue

• Serious illness during pregnancy that may affect hormone support

Outside of these scenarios, routine supplementation is rarely required.

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Why supplementation after day 100 is usually unnecessary

After this point:

• the placenta produces sufficient progestins

• ovarian progesterone becomes less relevant

• blood progesterone levels are no longer a reliable indicator

This is where many mares continue to receive treatment without clear benefit.

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The cost of unnecessary supplementation

It adds up quickly.

Daily hormone therapy over several months can become a significant expense without improving outcomes.

More importantly, it creates a false sense of security that everything is being “covered”.

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Risks of routine progesterone use

One of the more overlooked issues:

If a mare loses the pregnancy while on progesterone, she may not show normal signs of returning to estrus.

This can delay detection of pregnancy loss.

That is why:

• regular pregnancy checks matter

• you should not rely on medication alone as reassurance

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Products used for supplementation

The main options used in practice:

• Altrenogest (Regu-Mate)

• Injectable progesterone formulations

These are the products with established use in mares.

Not all “hormone supplements” on the market are equivalent or evidence-based.

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Monitoring matters

If progesterone is used:

• monitor pregnancy with ultrasound

• reassess need regularly

• avoid continuing indefinitely without reason

Treatment should always be part of a plan, not a habit.

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When to involve your vet

You should speak to your vet if:

• you are considering starting progesterone

• your mare has a history of pregnancy loss

• there are concerns about illness during pregnancy

• you are unsure whether supplementation is still needed

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FAQ

Do all pregnant mares need progesterone

No. Most mares maintain pregnancy without supplementation.

Is progesterone needed after day 100

Usually not, as the placenta produces sufficient hormones.

Can progesterone prevent abortion

Only in specific cases where deficiency is confirmed or risk factors exist.

Should I keep giving it just to be safe

Not without veterinary guidance. Unnecessary use is common.

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Final thoughts

Progesterone is important. That part is not in question.

What has changed is how we use it.

Most mares do not need routine supplementation, especially later in pregnancy. Using it without a clear reason adds cost, complexity, and can mask problems rather than solve them.

The goal is not to do more. It is to do what is actually needed.

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Still unsure what to do next?

Breeding decisions can feel like they carry a lot of pressure.

Should you supplement?
Is this pregnancy at risk?
Are you doing the right thing?

If you are unsure, getting clear guidance early helps you avoid unnecessary treatment and focus on what actually matters.

The ASK A VET™ app gives you access to veterinary advice when you need it, helping you manage your mare with confidence.