Options for pregnancy testing a mare

The best result of a breeding programme: a healthy mare and foal.
The best result of a breeding programme: a healthy mare and foal.

What options do owners have in determining if their mare is pregnant? Dr Keith Henderson explains the options to Horsetalk.

Is my mare pregnant? It’s a question any breeder will be asking themselves in the days and weeks after their mare has been put to a stallion.

Few owners will be content playing the waiting game. With an average gestation period of 342 days, it could be a long and, ultimately, disappointing wait to discover a mare is empty.

Determining whether a mare is in foal without testing can be difficult, even quite late in pregnancy.

Most owners will want to know the status of their mare as soon as practically possible. Today, there are more options than ever, including home-based kits that can determine pregnancy by the testing of urine or blood.

Veterinarians can also conduct an examination to determine whether a mare is pregnant, or take a blood sample for sending off for laboratory analysis.

The graph shows how urine samples from pregnant mares have much higher concentrations of oestrone sulphate than non-pregnant mares.
The graph shows how urine samples from pregnant mares have much higher concentrations of oestrone sulphate than non-pregnant mares.

How owners proceed will depend upon their budget and the number of broodmares in their care. Having a vet turn up to check multiple mares will naturally be more cost-effective than just one or two.

Home-based tests have the potential to save money, but it is important owners understand their application and ensure they are using the test kit in the optimal window.

Different tests measure different substances associated with pregnancy, and the circulating levels of these change throughout gestation. Incorrect application of a pregnancy test can result in an incorrect diagnosis.

Dr Keith Henderson, a research scientist for AgResearch’s Reproduction Group, based at the Hopkirk Research Institute at New Zealand’s Massey University, is an expert in the pregnancy testing of mares.

He has so far led the development of two pregnancy tests for mares in current use, and is continuing work in the field.

His laboratory is responsible for all the blood oestrone sulphate testing in New Zealand to determine mare pregnancy status. It recently produced a urinary version of this test called Wee-Foal-Checker which allows breeders to pregnancy test mares themselves with a non-invasive test.

Dr Henderson says accurately determining whether a mare is pregnant is an important part of broodmare management.

“Given the relatively short breeding season and the desire by owners to get mares in foal early on, it is important to be able to determine as early as possible whether or not conception has occurred,” he says.

Many breeders also want to verify the status of mares later in gestation – five to seven months into pregnancy – to ensure the mare hasn’t slipped her foal.

“Unfortunately, in contrast to the human situation, it is not straightforward, even late in gestation, to gauge mare pregnancy status by simple visual observation,” he says.

With some mares there is little doubt that they are in foal. But some mares seem to be able to fool their owners into thinking they are indeed in foal when they are not.

With some mares there is little doubt that they are in foal. But some mares seem to be able to fool their owners into thinking they are indeed in foal when they are not.
With some mares there is little doubt that they are in foal. But some mares seem to be able to fool their owners into thinking they are indeed in foal when they are not.

So what options do owners have? There are several non-chemical and chemical methods available, suitable for different stages of pregnancy.

Non-chemical methods

Behavioural changes: The mare’s behaviour towards a stallion can be a good indicator. From about 14 days after serving, a pregnant mare should reject a stallion’s advances. That is, they will not display the characteristic behaviour, which includes welcoming his advances and “showing” their rear.

If a mare does not begin her oestrus cycle within 21 days of being served, it can be assumed she is pregnant. Experienced breeders may also notice other subtle changes in the behaviour of their mare(s) as she starts to “bloom” in pregnancy.

Transrectal palpation: A veterinarian puts a hand in the rectum and feels the uterus for signs of pregnancy. These include uterine tone, the shape of the uterus, and the presence and size of the small sac containing the foetus, called the amnionic vesicle.

This method can be used from as early as 16 to 19 days of pregnancy, but it does require considerable expertise on the part of the veterinarian.

Ultrasound: A vet places a probe in the rectum of the mare and sound waves are used to generate a picture of the uterus, the foetus and placenta. It can also detect the foetal heartbeat.

Ultrasound can be used to detect pregnancy from about day 16, and can even reveal gender from about 55 to 70 days. Like palpation, ultrasound requires a highly skilled operator, and the equipment itself is expensive.

Chemical methods

While transrectal palpation and ultrasound are accurate ways of determining pregnancy, there are many times when the mare is too irritable to be examined, or its rectum is simply too small for a manual examination. Blood samples can be taken from the mare by a veterinarian and sent to a laboratory to be tested for the presence of chemical hormones associated with pregnancy.

The two hormones most commonly measured are pregnant mare serum gonadotrophin (PMSG), which is now often called equine chorionic gonadotrophin (eCG), and oestrone sulphate.

Pregnancy testing by measuring PMSG: This is best performed between 40 and 100 days after breeding. PMSG is a protein produced by the endometrial cups – a group of cells from the embryo that temporarily colonise the lining of the mare’s uterus.

The endometrial cups last for 60 to 100 days. They form between about days 36 to 38 of pregnancy and the PMSG they secrete is first detectable in the mare’s blood from about day 40.

Concentrations then rise quickly and peak around day 70. Levels in the blood gradually fall as the cups start to regress.

By about day 130, PMSG production has ceased, and it is no longer detectable in blood. It is generally not recommended to test for PMSG after 120 days of pregnancy as levels can be very low in some mares by then, and thus difficult to detect.

11 months can be a long wait for no foal.
11 months can be a long wait for no foal.

Normally, a veterinarian takes a blood sample from the mare being pregnancy tested and sends it off to veterinary diagnostic laboratory for measurement of the PMSG concentrations.

However, there is also a test kit available called Pregnamare, which allows breeders to test for PMSG themselves without the need for a veterinarian. The test requires a small quantity of blood taken from the nose or lip of the mare.

Dr Henderson says one problem with the PMSG blood test is that it can return a “false positive” result if the mare has been pregnant but slips the foetus.

This happens because the endometrial cups remain functional, and continue to produce PMSG for a time, even if the foetus dies.

If the foetus is lost after the cups have formed, the high concentrations of PMSG also act to suppress the normal breeding cycle of non-pregnant mares.

It is only when the cups have ceased to function, and PMSG has disappeared from the mare’s blood, that she resumes displaying oestrus behaviour and ovulation recommences.

It is advisable to re-test a mare diagnosed as pregnant by a PMSG test some time after 100 days after breeding to check that the foetus is intact.

Dr Henderson says it is best to use a test for oestrone sulphate after the 100-day mark.

“This oestrogen is produced by the foetal-placental unit, and measuring its blood concentration is a well-established, reliable and accurate method for pregnancy-testing mares from 100 days post-breeding right through to the expected time of foaling,” Dr Henderson says.

Oestrone sulphate is a hormone present in small amounts in the blood of non-pregnant mares.

From about the 40th day of pregnancy, the foetal-placental unit of pregnant mares start to produce increasing amounts of oestrone sulphate, which enters the mare’s blood and urine. By day 100 of pregnancy, the mare’s blood concentrations of oestrone sulphate are consistently at least twice, and generally more than five times, the highest concentrations found in non-pregnant mares.

Elevated levels are totally dependent on the presence of a live foetus. Levels remain elevated for the rest of gestation unless the foetus dies, whereupon concentrations fall within a few days to levels normally found in non-pregnant mares.

“Measuring oestrone sulphate concentrations in a blood sample taken 100 or more days after breeding is a very accurate method of determining pregnancy status,” Dr Henderson says.

“Unlike the PMSG test, it will not return a false positive diagnosis.

“Measuring blood oestrone sulphate concentrations earlier than 100 days post-breeding is not recommended, as a false negative diagnosis could arise if the pregnant mare still has blood oestrone sulphate concentrations in the upper part of the normal non-pregnant range.

“However, the likelihood of returning a false negative lessens the closer to 100 days post-breeding the mare is,” he says.

Vets follow the same procedure as they would a PMSG test. They take a blood sample from the mare and send it to a veterinary diagnostic laboratory for measurement of the oestrone sulphate concentrations. In New Zealand, it is Dr Henderson’s laboratory which conducts this test, with other diagnostic laboratories also forwarding blood samples they receive for this anlaysis.

Urine-based pregnancy testing

Oestrone sulphate produced by the foetal-placental unit not only finds its way into the maternal bloodstream, but also into the mare’s urine.

Urine-based oestrone sulphate tests have been available for some time, but have required the collected sample to be sent by the breeder to a laboratory for testing.

Recently, Dr Henderson and his research associate, Kim Wearne, developed a mare-side version of the laboratory urine test called Wee-Foal-Checker. This 10 minute, $30 test allows breeders themselves to non-invasively pregnancy test mares with the same accuracy as the laboratory test. It requires less than 1ml of urine and can be used from 110 to 300 days post-breeding.

He said the decision to develop the 10-minute home test acknowledged the fact that engaging a veterinarian to do a test can be expensive.

“The cost factor is often enough to discourage owners from pregnancy-testing their mare, in particular a confirmation test later in gestation, after an early pregnancy test.”

They are now working on a urine-based test that can be used from around 40 days after breeding – one that will measure PSMG levels.

Important considerations

Horse owners have the options of having a veterinarian pregnancy test their mare or undertaking the testing themselves. If owners decide to do it themselves, they have the option of blood or urine-based tests.

Owners undertaking home tests must be sure they understand the nature of the test and use it only within the time-frame recommended by the test’s manufacturers. To do otherwise risks an incorrect result.

Some might consider home-testing to be an affordable option for, say, a second test to ensure the mare has not slipped the foetus in the first 100 or so days.

Others will prefer to have an early veterinary check to rule out the possibility their mare is carrying twins.

Home-based test kits are not about to put equine veterinarians out of business. Used sensibly, they provide horse owners with an opportunity not only to save some money but, with a later second test, prove that their mare’s gently rounding tummy is because of a foal – and not the spring grass.

Eleven months is a long time to wait to be ultimately disappointed.

How to collect urine

A home-based urine test presents only one issue for horse owners: how to collect the sample.

Dr Keith Henderson says some breeders find it easy while others struggle.

“Many owners,” he says, “know the habits of their mares and are aware that they will pass a urine sample at a regular time, for example after a feed or after exercise.

“Collection of a urine sample under these circumstances is not so difficult. Not all the urine volume voided needs to be collected. Even if most falls on the ground, a sample can still be collected by dropping a paper towel on to it and recovering the urine.”

Other ways of encouraging urination are to put the mare into a stable stall with fresh straw and/or to whistle at her.

Another method is a simple device devised by Gill Booth, of KeriKeri.

The pictures reveal its construction – a collection bottle held in a netting container which attaches to the mare’s cover. Mares, he says, are barely aware of its presence.

The best collection bottle has been found to be a 2-litre juice container cut in half. Using a little cotton wool in the container ensures some urine will be retained, even if there is a spillage before you have a chance to collect it.


Pregnancy testing miniature horses

The small size of miniature horses makes it impractical to pregnancy test them by traditional methods used for full-size mares such as manual palpation via the rectum, or with ultrasound technology using a rectal probe.

Measurement of chemical hormones associated with pregnancy, which are found in the blood and/or urine of pregnant mares, provides an alternative and reliable means of pregnancy testing miniature horses, and monitoring of pregnancy throughout the gestation period.

The two chemical hormones commonly measured to provide information on the pregnancy status of miniature mares are pregnant mare serum gonadotrophin (PMSG), which is frequently now called chorionic gonadotrophin (eCG), and oestrone sulphate (see main story for more information).

For determining pregnancy status between 40 and 100 days after mating, measuring blood PMSG concentrations is recommended.

However, mares returning a positive PMSG test should be re-tested with a blood oestrone sulphate test after 100 days, or with the home-based urine Wee-Foal-Checker test after 110 days, to ensure the original result was not a false positive PMSG result.

Elevated levels of oestrone sulphate indicate the presence of a live foetus.

Dr Henderson says measuring blood or urinary concentrations of oestrone sulphate is a very accurate way of determining pregnancy status from approximately 100 days to 300 days post breeding.

First published on Horsetalk on May 12, 2008



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11 thoughts on “Options for pregnancy testing a mare

  • April 24, 2014 at 10:48 am

    Can you preg check a mare with a swine preg checker

  • August 2, 2014 at 1:37 am

    good & simple article

  • December 4, 2014 at 3:35 pm

    my mare was bread on june 1st2014 we had our first vet check at 28 days and another sonagram two months later fetus was good and another on oct 28 2014 at 151 days we did not see the fetus but it was really thick lining in side she has not gone in heat again could she still be pregnate and we just couldnt see it

    • January 31, 2015 at 6:34 am

      We are in the same boat…our mare was field bred June 10 27th and then on July 22 (that i observed myself)she never returned to heat after july breeding…We did two 38s they were positive…just retested with a 120 at 181 days and it was negative…But she looks pregnant…We are so confused….

  • September 5, 2015 at 4:20 pm

    i have a mare since the end of may and she has a min stallion in with her and we just got a gelding and she hasnt gone in heat i was told she could be pregnant what do you think thank you Ann whidden

    • July 4, 2020 at 2:30 am

      That’s was good but what is it called is there a horse instement or is it something else.

  • August 2, 2016 at 1:15 pm

    My mare was supposed to have been bred on 7/23/15. It is now August and she has not foaled. What would be the besg test to use for her? I appreciate any opinions.

  • October 27, 2016 at 3:11 pm

    i have a mini mare, I thought she was with foal we had a blood test performed by my vet . It came back negative for pregnancy my question is would her blood results be the same numbers as a full size horse? she still looks like she is going to have a foal .

  • October 27, 2016 at 3:15 pm

    thank you for your help

  • March 11, 2020 at 9:16 am

    I just used your urine pregnancy test. Now we are guessing if it really is light green or yellow. Can you help?


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