Does My Pregnant Mare Need Progesterone?

By Dr Duncan Houston

Progesterone support is one of the most common “just in case” treatments in mare reproduction.

That is understandable. Losing an early pregnancy is frustrating, expensive and emotionally brutal. If a daily medication might help protect the pregnancy, many owners feel it is safer to use it than risk doing nothing.

But progesterone is not magic glue for a pregnancy. Some mares genuinely need progestogen support. Many do not. The skill is knowing which situation you are dealing with before you spend money, create handling risks or gain false reassurance.

Quick Answer

A pregnant mare may need progesterone or progestin support if she has documented low progesterone early in pregnancy, luteal insufficiency, loss of luteal support, ovariectomy, prostaglandin exposure, or a specific high-risk pregnancy condition being managed by a reproduction veterinarian.

Most normal pregnant mares do not need routine supplementation. After around 110 to 120 days, the feto-placental unit produces other progestagens that maintain pregnancy, so a low standard progesterone result later in gestation does not automatically mean the mare is unsupported. (vetmedbiosci.colostate.edu)

Progesterone, Progestin and Progestogen: What Is the Difference?

These words are often used loosely, but they are not identical.

Progesterone is the natural hormone produced by luteal tissue after ovulation.

Altrenogest, commonly known by the product name Regu-Mate®, is a synthetic progestin. It acts on progesterone receptors but is not the same molecule as natural progesterone.

Progestogen is the broader term covering progesterone and substances with progesterone-like effects.

This matters because not every product marketed as “progesterone support” has been proven to maintain equine pregnancy. Altrenogest and injectable progesterone have evidence behind them in mares, while several other synthetic progestins have failed to maintain pregnancy in experimental settings. (clinicaltheriogenology.net)

What Does Progesterone Do in a Mare?

Progesterone prepares and maintains the uterus for pregnancy.

In early pregnancy, it helps:

• Maintain uterine tone

• Support the endometrium

• Keep the cervix closed

• Reduce uterine contractility

• Support early conceptus development

• Maintain pregnancy until other progestagens take over

The mare’s early pregnancy is heavily dependent on luteal tissue. If the corpus luteum fails too early or progesterone falls below a functional level, the embryo may not develop normally and pregnancy loss becomes more likely. A 2023 study found that reduced progesterone during early pregnancy delayed conceptus development and increased pregnancy loss. (PubMed)

The Pregnancy Timeline: Why Timing Changes Everything

The decision to supplement depends heavily on the stage of pregnancy.

Stage of pregnancy	Main source of pregnancy support	Why it matters
Ovulation to around day 35	Primary corpus luteum	This is the stage where low progesterone matters most
Around days 35 to 100	Primary and secondary corpora lutea stimulated by eCG	Several luteal structures may contribute progestogen support
Around days 70 to 120	Transition towards feto-placental progestagen production	Standard progesterone results become less straightforward
After about 110 to 120 days	Feto-placental progestagens, including DHP and related pregnanes	Low ordinary progesterone alone does not necessarily mean the pregnancy is unsupported

The major modern correction is that late equine pregnancy is not maintained simply by the same progesterone pattern seen in early pregnancy. Research has shown that 5α-dihydroprogesterone, or DHP, can activate the equine progesterone receptor and helps define a second phase of pregnancy support. UC Davis recommends stopping routine supplemental support when DHP is expected to exceed progesterone at approximately 110 to 120 days, unless there is another clinical reason to continue. (PMC)

When Is Progesterone Supplementation Actually Indicated?

1. Documented Low Progesterone Early in Pregnancy

This is the cleanest indication.

Colorado State University’s Equine Reproduction Laboratory states that if a pregnant mare’s progesterone is below 4.0 ng/mL on analysis, supplementation is indicated to help maintain the pregnancy. (vetmedbiosci.colostate.edu)

That said, one low result should still trigger the question:

Why is progesterone low?

Possible reasons include:

• Poor corpus luteum function

• Premature luteolysis

• Uterine inflammation

• Endometritis

• Prostaglandin exposure

• Incorrect ovulation date

• Laboratory or timing issue

• A failing pregnancy

Treating low progesterone without looking for the cause can hide the real problem. If the uterus is inflamed or infected, simply adding progestin may not fix the pregnancy risk. Uterine infection decisions should be based on culture, cytology, ultrasound findings and reproductive history rather than assumption.

2. Previous Early Pregnancy Loss With Evidence of Low Progesterone

A mare with repeated early pregnancy loss may be a candidate for progesterone monitoring and targeted supplementation.

This is different from saying every mare that lost a pregnancy once needs Regu-Mate® forever.

The work-up should include:

• Accurate ovulation dating

• Early pregnancy ultrasound

• Twin check

• Progesterone measurement

• Uterine fluid assessment

• Culture and cytology where indicated

• Semen quality and timing review

• Consideration of endometrial biopsy in older or repeat-breeder mares

Recent research supports the idea that low early progesterone can be detrimental to conceptus and endometrial development, but it does not prove that every early loss is caused by progesterone deficiency. (PubMed)

3. Luteolysis or Prostaglandin Exposure

If a pregnant mare is accidentally given prostaglandin or experiences premature luteal regression, progestogen support may be used to try to maintain pregnancy.

The FDA information summary for altrenogest includes data suggesting altrenogest may be therapeutically useful when prostaglandin-induced corpus luteum suppression creates progesterone deficiency, provided it is used at an appropriate dose and duration. (Animal Drugs at FDA)

This is a genuine veterinary situation, not a casual supplement decision.

4. Ovariectomized Mares or Special Embryo Transfer Situations

Mares without ovaries cannot provide normal luteal progesterone support.

Research has shown that ovariectomized mares treated with altrenogest can establish pregnancy after embryo transfer. (PubMed)

In these cases, progestogen treatment is not “extra support.” It is replacing a missing biological source of pregnancy maintenance.

5. Selected High-Risk Pregnancies

Altrenogest is sometimes included in treatment plans for mares with placentitis or other high-risk pregnancies, usually alongside antimicrobials, anti-inflammatories and close ultrasound monitoring.

The evidence is mixed. One experimental placentitis study found better foal viability with combined trimethoprim-sulfamethoxazole, pentoxifylline and altrenogest therapy, but other work has questioned whether altrenogest reliably changes outcomes in disturbed late pregnancies. (PubMed)

The practical message is:

Late-pregnancy altrenogest should be case-based, not automatic.

When Is Progesterone Usually Not Needed?

Progesterone supplementation is usually not justified when:

• The mare has a normal early progesterone concentration

• The pregnancy is viable on ultrasound

• There is no history of early loss

• There is no luteal problem

• There is no prostaglandin exposure

• There is no high-risk pregnancy diagnosis

• The mare is already beyond the luteo-placental transition with no clinical concern

• The pregnancy has already been lost

This is where money often disappears.

A normal pregnant mare does not become safer simply because a bottle of altrenogest appears in the feed room. If there is no diagnosis, no low progesterone and no high-risk condition, supplementation may add cost and handling risk without improving the outcome.

Why Late Pregnancy Progesterone Testing Can Mislead Owners

This is a common trap.

In early pregnancy, progesterone testing can help evaluate luteal function. Later in pregnancy, other progestagens maintain the pregnancy, and standard progesterone assays may not measure the biologically important pregnanes that have taken over.

Cornell’s reproductive testing guidance states that progesterone should not be used for monitoring mid and late trimesters of equine pregnancy because other progestins maintain the pregnancy. It also notes that Regu-Mate® does not cross-react with its progesterone assay. (Cornell Vet College)

So if someone says:

“My mare is 150 days pregnant and her progesterone is low.”

The right response is not automatically:

“She must need Regu-Mate®.”

The better response is:

What assay was used, what stage of pregnancy is she in, is the fetus viable, what do the placenta and uterus look like, and are late-pregnancy progestagens being assessed appropriately?

How Should Pregnancy Be Monitored Instead?

Pregnancy support decisions should be built around ultrasound and clinical findings, not progesterone alone.

A common pregnancy-check schedule includes:

Timing after ovulation	Main purpose
Days 14 to 18	Confirm pregnancy and check for twins
Days 25 to 30	Confirm embryo development and heartbeat
Days 40 to 60	Confirm ongoing viability
Later pregnancy	Assess fetal wellbeing, placenta and risk factors when indicated

Merck Veterinary Manual lists days 14 to 18 for pregnancy and twin detection, days 25 to 30 for embryo development and heartbeat, and later checks for ongoing fetal development. (Merck Veterinary Manual)

If a mare is on supplementation, pregnancy checks become even more important because behavioural return to estrus may be suppressed.

Does Regu-Mate® Hide Pregnancy Loss?

It can delay obvious behavioural clues.

If a mare loses a pregnancy while receiving altrenogest, she may not show normal heat behaviour while treatment continues. That does not mean the pregnancy is still alive.

This is why mares on progestogen support need scheduled ultrasound checks. Do not use the absence of estrus as proof that everything is fine.

In practice, this is one of the most common mistakes: the owner keeps dosing because the mare “has not come back into season,” but no one has confirmed a viable embryo or fetus.

Which Products Are Proven to Work in Mares?

The most commonly used options are:

• Oral altrenogest

• Daily injectable natural progesterone

• Long-acting injectable progesterone preparations

Colorado State University lists altrenogest, natural progesterone by intramuscular injection, short-acting daily progesterone and long-acting weekly progesterone as options for pregnancy support when progesterone is low. (vetmedbiosci.colostate.edu)

The important warning is that not every synthetic progestin works in mares. A review of progestin use reports that medroxyprogesterone acetate, hydroxyprogesterone hexanoate, norgestomet and megestrol acetate were unable to maintain pregnancy in experimental models, while altrenogest and progesterone in oil had more consistent biological effect. (IVIS)

If a product is cheaper but not proven to support equine pregnancy, it is not actually a bargain. It is just a cheaper uncertainty.

Handling Risk: Altrenogest Is Absorbed Through Human Skin

This part matters because owners and barn staff handle the drug every day.

The FDA warns that altrenogest products can cause reproductive system disorders and other adverse effects in people who are exposed. Product safety information also states that Regu-Mate® is absorbed through unbroken skin, that pregnant women or women who may be pregnant should not handle it, and that vinyl, neoprene or nitrile gloves should be used because latex gloves are not protective. (U.S. Food and Drug Administration)

Safe handling means:

• Do not allow pregnant people to administer it

• Wear appropriate gloves

• Avoid skin contact

• Clean spills immediately

• Keep dosing equipment separate

• Prevent children and unprotected staff from touching contaminated surfaces

• Do not contaminate feed bins, door latches or buckets

This is not a harmless syrup. It is a hormonal medication that happens to come in a bottle.

Potential Downsides of Unnecessary Supplementation

Progesterone or progestin support is useful when indicated, but unnecessary use has downsides.

Cost

Daily medication over weeks or months adds up quickly, especially across multiple mares.

False Reassurance

Supplementation does not prove the embryo is alive, the uterus is healthy, the placenta is normal or the foal is safe.

Masking Return to Estrus

A mare may not show heat even if pregnancy has failed while she remains on altrenogest.

Human Exposure Risk

Altrenogest requires careful handling and is not safe for all handlers.

Inappropriate Late-Pregnancy Confidence

Late pregnancy loss is often related to placental disease, infection, fetal compromise or systemic illness, not simple progesterone deficiency.

Possible Periparturient Concerns

Late-pregnancy altrenogest has not consistently prevented parturition or pregnancy loss in disturbed pregnancies. A study in mares with experimentally disturbed pregnancies concluded its effectiveness in preventing abortion or preterm foaling should be questioned. (PubMed)

Does a Sick Pregnant Mare Need Progesterone?

Sometimes, but the illness matters more than the label.

A pregnant mare with colic, fever, placentitis, systemic inflammation or severe stress may be at risk of pregnancy loss. Progestogen support may be considered as part of a broader plan, but it should never distract from diagnosing and treating the underlying problem.

A sick pregnant mare may need:

• Physical examination

• Ultrasound

• Bloodwork

• Fetal monitoring

• Placental assessment

• Antimicrobials where indicated

• Anti-inflammatory treatment

• Fluids

• Surgery or referral in severe cases

• Carefully selected pregnancy support

Do not respond to illness in a pregnant mare by only adding Regu-Mate® and hoping the uterus behaves.

The uterus is not the whole patient.

How Worried Should You Be?

Low Concern

The mare:

• Has normal early progesterone

• Has a viable pregnancy on ultrasound

• Has no history of early pregnancy loss

• Has no uterine fluid or infection

• Has no systemic illness

• Is not receiving prostaglandin or high-risk medications

What to do: routine pregnancy monitoring is usually enough. Progesterone support may not be needed.

Moderate Concern

The mare:

• Has a previous early loss

• Has one borderline progesterone result

• Has uncertain ovulation timing

• Has a questionable corpus luteum

• Has mild uterine fluid

• Is older or a repeat breeder

What to do: recheck progesterone and pregnancy viability, assess the uterus, review breeding timing and decide whether targeted support is justified.

High Concern

The mare:

• Has documented low progesterone early in pregnancy

• Has luteolysis

• Has prostaglandin exposure

• Has recurrent early loss with supportive findings

• Is ovariectomized or hormonally dependent

• Has a high-risk pregnancy under specialist care

What to do: progestogen support may be indicated, but it should be paired with scheduled ultrasound checks and a clear stop or review date.

Critical

The mare:

• Has vaginal bleeding

• Has foul vulvar discharge

• Develops fever or marked depression

• Has colic

• Shows premature udder development

• Leaks milk before term

• Has suspected abortion

• Shows laminitis signs

• Collapses or deteriorates rapidly

What to do: this is not a supplement question. Seek urgent veterinary care.

When Is This an Emergency?

Call your veterinarian urgently if a pregnant mare develops:

• Colic signs

• Fever

• Depression or refusal to eat

• Vaginal bleeding

• Vulvar discharge

• Premature udder development

• Milk leakage before term

• Suspected abortion

• Straining or signs of early labour

• Severe lameness or laminitis signs

• Rapid deterioration

• Weakness or collapse

Progestogen medication does not treat dystocia, placentitis, abortion, colic, sepsis or laminitis by itself.

If the mare is unwell, treat the mare first.

What Else Causes Early Pregnancy Loss?

Low progesterone is only one possible reason.

Important causes include:

• Twin pregnancy

• Poor embryo quality

• Age-related oocyte decline

• Endometritis

• Persistent uterine fluid

• Poor uterine clearance

• Endometrial fibrosis

• Cervical dysfunction

• Poor semen quality

• Inaccurate breeding timing

• Failure of maternal recognition

• Systemic illness

• Severe stress or transport in susceptible mares

• Hormonal disruption

• Placentitis later in pregnancy

A mare that repeatedly loses pregnancies deserves a reproductive diagnosis, not just a longer course of altrenogest.

The most expensive plan is often repeating the same unsupported treatment each season while the actual cause quietly stays in the mare.

What Should You Do Before Starting Progesterone?

1. Confirm Pregnancy and Ovulation Date

Do not supplement blindly because a mare “might be in foal.”

Confirm:

• Ovulation date

• Pregnancy status

• Number of embryos

• Embryo size

• Uterine findings

• Corpus luteum presence

2. Check for Twins

Twin pregnancies are a major reason for early loss in mares and must be managed early. Progesterone support will not make a twin pregnancy safe.

3. Measure Progesterone When It Is Clinically Useful

Early testing is most useful when:

• There is previous early pregnancy loss

• The corpus luteum looks abnormal

• Pregnancy is valuable or high risk

• Prostaglandin exposure occurred

• Luteal insufficiency is suspected

4. Investigate the Uterus

Before assuming progesterone is the problem, check for:

• Uterine fluid

• Endometritis

• Poor clearance

• Cervical dysfunction

• Culture and cytology results where indicated

5. Choose the Correct Product

Use products with evidence in mares, under veterinary direction.

Do not substitute random hormonal products because they are cheaper or easier to obtain.

6. Set a Recheck Schedule

Plan rechecks for:

• Pregnancy viability

• Heartbeat

• Progesterone where useful

• Uterine or placental health

• Treatment duration

• When to stop or continue

7. Use Safe Handling

Assign a suitable handler and use proper gloves.

When Can Supplementation Be Stopped?

There is no single answer for every mare.

Possible stopping points include:

• Once progesterone normalises and pregnancy is stable

• Around day 45 to 60 in selected early-support cases

• Around day 100 to 120 when feto-placental progestagens take over

• Later only if a specific high-risk pregnancy diagnosis requires continued therapy

UC Davis suggests the safest practice is to stop supplemental support when DHP is expected to exceed progesterone at about 110 to 120 days, unless there is a specific clinical reason to continue or reintroduce it, such as placentitis. (vetmed.ucdavis.edu)

Do not stop suddenly without discussing the case with the veterinarian, especially in mares that are known to be dependent on support.

Common Mistakes Owners Make

Giving Regu-Mate® to Every Pregnant Mare

Routine treatment without a diagnosis may add cost and risk without improving outcome.

Treating a Number Instead of the Mare

A progesterone result must be interpreted with pregnancy stage, ovulation date, ultrasound findings and assay type.

Panicking Over Low Late-Pregnancy Progesterone

Standard progesterone testing is not the best tool for mid or late pregnancy because other progestagens maintain the pregnancy. (Cornell Vet College)

Forgetting to Recheck the Pregnancy

The mare may not return to heat while on altrenogest, even if the pregnancy has failed.

Using the Wrong Progestin

Several synthetic progestins are not reliable for maintaining pregnancy in mares. (IVIS)

Ignoring Uterine Disease

A mare with endometritis, fluid or poor uterine clearance needs diagnosis and treatment of the uterine problem, not just hormonal cover.

Letting Pregnant Handlers Administer Altrenogest

Pregnant women or women who may be pregnant should not handle Regu-Mate®. (Merck Animal Health USA)

Assuming Supplementation Prevents Abortion

It does not prevent every pregnancy loss, especially when placental disease, infection, fetal compromise or severe maternal illness is present.

How Can Pregnancy Loss Risk Be Reduced?

Not every loss can be prevented, but risk can be reduced through better reproductive management.

Practical steps include:

• Breed mares in appropriate body condition

• Confirm ovulation timing accurately

• Use good-quality semen

• Check pregnancy early

• Check for twins at days 14 to 18

• Recheck heartbeat around days 25 to 30

• Assess progesterone when there is a real indication

• Investigate repeat pregnancy loss properly

• Treat uterine infection only when supported by evidence

• Monitor older and high-risk mares more closely

• Screen for placentitis when signs or risk factors are present

• Avoid unnecessary medications in pregnancy

• Keep accurate records across seasons

The best pregnancy support is not always another medication.

Often, it is better timing, better diagnostics and fewer assumptions.

Will My Mare Be Okay?

Most mares with normal luteal function and a viable pregnancy do not need progesterone support and continue normally.

The outlook is better when:

• Low progesterone is identified early

• The pregnancy is viable

• Support is started for a real indication

• The uterus is healthy

• Twins have been ruled out

• The mare is rechecked regularly

• The underlying cause is addressed

The outlook becomes more guarded when:

• Pregnancy loss has already started

• The uterus is inflamed or infected

• Twin pregnancy is present

• The mare is systemically ill

• Placentitis is present

• Supplementation is started late without diagnosis

• The mare has repeated losses despite treatment

Progesterone can be very useful in the right mare.

It is not a substitute for knowing what is actually wrong.

FAQs About Progesterone in Pregnant Mares

Does every pregnant mare need Regu-Mate®?

No. Most normal pregnant mares do not need routine Regu-Mate®. It is most useful when there is documented low progesterone, luteal insufficiency, prostaglandin exposure, ovariectomy or a specific high-risk pregnancy indication.

What progesterone level is too low in early pregnancy?

Colorado State University’s Equine Reproduction Laboratory uses less than 4.0 ng/mL as a level where progesterone supplementation is indicated to help maintain pregnancy. The result should still be interpreted with ovulation date, ultrasound findings and the mare’s history. (vetmedbiosci.colostate.edu)

Can progesterone prevent abortion in late pregnancy?

Not reliably by itself. Late pregnancy losses are often linked to placentitis, fetal compromise, systemic illness or other problems. Altrenogest may be part of a specialist treatment plan, but it is not guaranteed abortion prevention.

Can I stop progesterone at 100 days?

Many mares can stop around the luteo-placental transition, often by about 110 to 120 days, when feto-placental progestagens are expected to maintain pregnancy. The decision should be made with your veterinarian, especially in high-risk mares. (vetmed.ucdavis.edu)

Is altrenogest safe for people to handle?

It requires strict handling precautions. The FDA warns that altrenogest exposure can cause adverse effects in people, and product guidance states pregnant women or women who may be pregnant should not handle Regu-Mate®. (U.S. Food and Drug Administration)

Final Thoughts

Progesterone support in mares is not good or bad by itself.

It is powerful when used for the right reason.

The right reasons include documented early progesterone deficiency, luteal support failure, prostaglandin exposure, ovariectomized recipients and selected high-risk pregnancies under veterinary care.

The wrong reason is fear alone.

The most important questions are:

• Is the mare actually pregnant?

• How many days post-ovulation is she?

• Is the embryo or fetus viable?

• Is progesterone low at a stage where that result matters?

• Is the uterus healthy?

• Is there a history that supports supplementation?

• What product is being used?

• When will the mare be rechecked?

• When should treatment stop?

Used properly, progesterone support can help protect a pregnancy that genuinely needs help.

Used automatically, it can waste money, hide pregnancy loss, expose handlers to risk and distract from the real cause of reproductive failure.

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If your mare is pregnant, has a low progesterone result, is on Regu-Mate®, or has a history of early pregnancy loss, ASK A VET™ can help you organise the timeline, test results and questions to discuss with your equine reproduction veterinarian.