Monitoring Pregnancy

Preparations for breeding and the act of getting a mare bred involve dedicated time and commitment by a horse owner. Then it seems as if all that needs to be done is to sit back and wait for the foaling date. But this is not quite so; it is important to stay on top of events throughout the mare's course of pregnancy. Development of an equine fetus takes about 11 months from conception to birth. During this period, there are critical times when careful observation might detect a problem in time to correct a serious issue that spells harm for mare and/or foal.

What is Normal Gestation?

Unless a mare has been bred in the pasture, the exact breeding dates are usually known. Lisa Metcalf, DVM, Dipl. ACT, a board-certified reproductive specialist in Oregon, notes what an owner can expect as parturition (delivery) approaches: "Normal gestation length in a mare ranges from 335-345 days. Foals delivered prior to 325 days gestation are considered premature. If a foal is delivered prior to 300 days of gestation, it is rarely compatible with life, even with the support of our advanced neonatal intensive care units."

Liz Scott, a DVM, practicing veterinarian at Idaho Equine Hospital with a special interest in equine reproduction and neonatology, concurs that as a general rule, at least a 325-day gestation is a minimum necessary for survival success. She elaborates: "A foal that is born premature with complications may end up being healthy if supported with aggressive veterinary care."

Metcalf says it is suspected that fillies are carried two days longer than colts on average, although in general, there is little difference in gestation length based on fetal gender.

Regarding whether a mare tends to follow a gestational length comparable to previous pregnancies, Metcalf notes, "Normal gestation lengths can vary by two weeks in the same mare."

Scott adds, "Mares are usually true to themselves, following a similar pattern in each pregnancy."

Both veterinarians agree that if a mare has had developmental placenta abnormalities in the past due to compromise of the endometrial architecture (lining of the uterus), this problem scenario is likely to be repeated, making the pregnancy a high-risk event.

Mammary development is a key feature that correlates with a mare's hormonal changes associated with impending foaling. Metcalf says udder development usually begins four to six weeks before foaling, although maiden mares might not show mammary development until just before parturition. "The mammary secretions normally turn from a sticky, serous secretion to a watery, 'skim-milk' consistency, to a thicker, somewhat yellowish colostrum just before foaling," she describes.

Metcalf also mentions observable physical changes in a mare's body. She says that in the month before foaling, the mare's abdomen will enlarge and become more pendulous (hanging downward). The paralumbar fossa (in the flank area) will look more sunken, and the gluteal musculature at her tailhead will relax. Due to stretching of the skin on either side of the tailhead that can cause mild pruritis (itching), the mare might want to rub her tail. Also during this pre-foaling time, she might become sedentary and spend more time lying down and even exhibit signs of mild colic.

But if a mare has excessive abdominal edema (fluid swelling), Scott urges she be evaluated immediately for an imminent pre-pubic tendon rupture or abdominal muscle rupture, both of which put the lives of the mare and foal in jeopardy.

Monitoring Devices

Metcalf has found a monitoring device she feels is helpful in determining foaling, noting, "Although many devices are available for alerting an attendant that foaling is imminent, for almost 20 years I have used a product designed by Chemetrics that is called Foal Watch. It tests the calcium level in the milk; when it rises above 250 ppm, there is a greater than 90% likelihood that the mare will foal within 24 hours."

Scott finds video monitoring systems especially useful. One that attaches to the television can cost less than $100, and an owner can roll over in bed every couple of hours to check the mare on the TV.

Scott says a baby monitor provides excellent audio feed to alert the owner to abnormal sounds from the foaling stall. She remarks, "A baby monitor allows an owner to hear the mare's water break, for example. Immediate veterinary response has improved survival of both mare and foal when a foal is lodged in an abnormal presentation in the birth canal."

Scott notes that vaginal alarms sewn into the vaginal entry are excellent alarms for most foalings, but won't always alert someone if a mare is in serious trouble. If a foal's feet don't enter the vaginal vault, the vaginal alarm might not go off; the mare could remain in trouble with no alert to the owner.

An alarm system that attaches to the halter is another alert device that signals when a mare lies down in lateral recumbency. Unfortunately, these alarms also go off if the mare lies down to take a nap or to roll and scratch, thereby eliciting some false alarms. And there is the occasional mare that foals in a standing position.

Impending Abortion

Not all mares make it to the expected date of parturition. High-risk situations increase the possibility of an abortion or a seriously sick foal. Scott identifies those conditions that pose a high risk to foal survival: "A mare that has had a septic foal in the past, or a dystocia (difficult birth), or has had a history of premature placental separation or red bag is a mare that needs to be monitored closely throughout her pregnancy. Serial ultrasound exams, especially in the latter months of gestation, are helpful to identify placental health or problems."

Metcalf comments, "Often, we have little or no warning that a mare is going to abort. Unless there is an outbreak of an infectious disease that causes abortion (rhinopneumonitis or equine viral arteritis), the most common stage of pregnancy loss is in the embryonic phase of development." And the most critical time of embryonic loss is within the first 50 days of pregnancy.

The best means to detect embryonic loss is to have your veterinarian perform rectal ultrasound exams to image the fetus. This is an excellent diagnostic tool to check for the presence or absence of an embryo, and to identify twin pregnancies.

Not all pregnancy loss occurs early in gestation; Metcalf says mid- to late-term abortion does occur. The most common cause is placentitis, an inflammation or infection within the placenta, she notes.

"It used to be twinning, but with the use of ultrasound early in gestation, twins are identified and usually resolved early on," Metcalf says.

"The most common clinical sign of placentitis is premature lactation," she notes. "It is not nearly as common to see a vaginal discharge. Diagnosis of placentitis can be somewhat challenging. A vaginal or cervical discharge makes it obvious and a bacterial culture can identify the causative organism, but often veterinarians use ultrasound to detect early stages. The mare rarely exhibits any signs of generalized illness."

Scott stresses the value of repeat ultrasound exams in the third trimester to measure placental thickness, evaluate the apposition (placement) of the placenta to the uterus, and to assess the character and volume of the fluid within the placenta. "Debris" is seen on ultrasound as gray or white material in the placental fluid; it is a signal of inflammation and infection.

Possible external signs of an impending abortion or potential foaling complications include any of the following, particularly if not in timing with an anticipated due date:

  • Premature udder development;
  • Premature lactation;
  • Vaginal discharge;
  • Any abnormal physical exam findings that indicate a systemic problem;
  • Softening of the vulvar lips and/or tailhead, signaling impending foaling;
  • Excessive abdominal edema or swelling that might portend pre-pubic tendon
    rupture, or excess abdominal size that might indicate hydrops (excess fetal fluids in the uterus), which could create dystocia.

Scott says, "These situations usually occur within the last trimester, so don't only rely on breeding date to estimate when there should be a concern about abortion."

When any abnormal signs are present, Metcalf says early detection is key. "A mare with placentitis is at high risk of abortion," she adds. "However, it is important to realize that a mare experiencing stress for any reason can be at risk. Physiological stressors such as disease, injury, surgery, etc., can cause abortion."

Physical abnormalities such as decreased appetite, increased digital pulses (suggestive of laminitis), or abnormal color and refill time of mucous membranes might be symptomatic of an unrelated problem that could have significant repercussions, including abortion, says Scott.

Both clinicians stress that the mare is the best incubator for the foal, whenever possible. "We'd rather treat the fetus in the mare rather than after foaling," adds Scott.

With this in mind, aggressive treatment should be implemented as soon as any abnormal signs are noticed.

"If a mare has had a history of a septic foal in the past or is showing any sign of impending abortion, she should be examined by a veterinarian who is accustomed to evaluating mares in late gestation with both rectal and transabdominal ultrasound," says Scott. "The clinician should be comfortable performing a thorough vaginal exam when appropriate, and be familiar with assessing the integrity of the cervix and identifying abnormal discharge."

A rectal exam checks the tightness of the cervix, the position of the foal, the size of the uterus, and movement of the fetus. In the past, an EKG was occasionally done on a foal to evaluate heart function, says Scott, but today, a transabdominal ultrasound performed after 150 days gestation accurately "sees" and measures the fetal heart beat while also checking the umbilical cord and looking for abnormal amnionic fluid or placental insufficiency.

When the placenta is affected by infection or decreased blood supply, it thickens with edema and can separate from the uterine wall. The placenta is critical for fetal survival because nutrients and oxygen are passed across it from mare to foal. Abnormal placental thickness or premature placental separation can be visualized with transabdominal ultrasound in 60% of mares about to abort due to placental insufficiency.

Ultrasound is also useful in measuring the size of the fetal aorta, the large vessel that sends blood out of the heart. The relative size of the aorta gives a rough estimate of the predicted size of the foal, alerting the practitioner and owner as to whether there could be problems in birthing related to size of the foal.

It is recommended to detect twins with rectal ultrasound at 14-16 days gestation, when there is the greatest success for reducing one of the twins and allowing the other to develop. After 90 days gestation, transabdominal ultrasound can usually identify twins, although Scott says on a rare occasion twins can be sitting on top of one another rather than side-by-side, making it difficult to identify them even with transabdominal ultrasound.

"Twin reduction can be accomplished at 116 days gestation by using a transabdominal ultrasound to accurately inject a potassium-containing compound into the fetal heart to eliminate one of the twins," notes Scott. She says that is a tricky and expensive procedure, with successful reduction of twins achieved about 50% of the time.

Hormonal Testing

Often veterinarians will conduct hormone testing of a high-risk mare. Scott relies on progesterone assays early in the pregnancy, at six to eight days post-ovulation and at 25-35 days post-ovulation, especially if the mare has had a history of early embryonic death and/or low progesterone levels. If progesterone levels are too low, the mare can be put on oral altrenogest to replace the progesterone that should be secreted by the corpus luteum on the ovary until the uterus takes over in controlling progesterone production by about Day 80. It is suggested to continue altrenogest supplementation until Day 100 as added insurance to overlap with the mare's normal hormonal output in early gestation.

Progestin levels rise slowly in the last three weeks of gestation, then plummet within 48 hours of foaling. Progestin levels can be evaluated at intervals preceding 310 days gestation, with blood samples taken from the mare every two to three days to obtain comparison levels of three samples. If progestins rise to more than 50% of the previous value (between 250-305 days), the fetus is considered stressed. During this period, if the progestin levels fall prematurely, then the fetus is in crisis and abortion might be imminent.

Another hormone evaluated by some practitioners is estrogen. Metcalf says, "The only hormonal testing that I perform during pregnancy is the estrone sulfate test (used to measure estrogenicity) when I am concerned about fetal well-being."

Estrogens are synthesized in the equine placenta from hormones produced by the gonads of the fetus. Estrogen levels rise at about Day 60 of pregnancy, peaking around Day 200, then slowly declining through the remainder of gestation. If estrogen levels decline markedly during Days 150 through 310, then the fetus might be in a stress state.

Currently, researchers are also looking into the value of relaxin testing since relaxin is another hormone produced by the fetus, identified in mare serum at about Day 80. Relaxin remains at high serum levels in the mare through gestation, and it is an important hormone in softening the pelvic ligaments preceding foaling. Researchers at Rutgers University are evaluating relaxin as a marker for fetal well-being and placental sufficiency. Their goal is to develop a useful diagnostic test for relaxin, applicable to high-risk pregnancies, and especially in areas at risk of fescue toxicity.

In cases where neonatal isoerythrolyis (NI, a cause of jaundice in foals) might be a concern, the mare's serum can be screened two to four weeks prior to the expected foaling date. If the test comes back suspicious, her serum should be screened again closer to foaling. Any foal from a mare with a positive screening test should not be allowed to suckle colostrum or milk for the first 24-48 hours. Although NI occurs in less than 2% of foals, when it does occur, it is often fatal. Maternal antibodies passed into the mare's colostrum will attack the red blood cells of the foal and create severe anemia and death. More information:

Delaying Early Parturition

An abortion can occur at any time. Scott outlines typical periods of fetal loss:

  • Twins abort seven months to term;
  • Equine herpesvirus aborts in the last four months;
  • Septic foals abort in the last two months;
  • Excess uterine scar tissue can cause abortion at any time due to limited nutrition as the foal grows; timing of abortion depends on the extent of uterine scar tissue; and
  • Equine viral arteritis causes abortion at any stage of pregnancy, within one to three weeks of exposure to the virus.

A mare showing signs of impending abortion, particularly when related to placentitis, needs attention. Metcalf says, "We know that oral altrenogest (Regumate) at a double dose is quite powerful in inhibiting loss of pregnancy in mares with endotoxemia. It has been found to be effective in cases of placentitis as well, especially coupled with the use of flunixin meglumine (Banamine), pentoxyfylline, and antibiotics that cross the placenta. Many veterinarians also use a tocolytic (inhibits uterine contractions) such as clenbuterol to quiet the uterus. Basically, the goal is to inhibit contractions, clear the infection, and modulate the inflammatory response.

"Flunixin meglumine is a prostaglandin inhibitor; pentoxyfylline has been shown to inhibit mediators (procytokines) involved in the inflammatory cascade as well as having a role in improving oxygenation of the placenta by enhancing red blood cell deformity (shape changeability) to more easily pass through blood vessels. Antibiotics such as sulfa-trimethoprim, penicillin, and gentamycin have been shown to cross the placenta and therefore represent the antibiotics most commonly used in initial phases of treatment."

The Mare's Foaling Place

To create the best foaling environment, Metcalf suggests if a mare is being transported to a foaling facility, that should be done four to six weeks before her due date so she can develop adequate antibodies in her colostrum unique to that farm's endemic pathogens.

She says every pregnant mare should be examined daily during the last month or two of gestation. This can be done briefly during a walk in the field or during turnout. This enables you to follow the progress of udder development, check for premature lactation, and look for waxing of the teats.

"I have always maintained that the ideal place to foal is in a grassy field in mild weather," says Metcalf. "However, due to so many factors, such as lack of space, lack of pasture, cold weather, and value of the foal, the mare often foals in a large stall, deeply bedded with wheat straw (not shavings), that is under 24-hour surveillance. In order for the mare to become used to being stalled, she should spend daily (or nightly) time in the stall during the few weeks prior to the expected foaling date."

Good hygiene is critical to foal health. One easy strategy can limit ingestion of microbes by the newborn foal when nursing: Washing of the mare's udder, perineum, and hind legs just prior to foaling. As a neonate seeks the udder in his first nursing attempts, he contacts all these body parts, licking and sucking contaminating feces and debris into his mouth. The more microbes ingested, the greater the likelihood that some will cross the intestinal barrier to create a systemic or joint infection. During the weeks preceding foaling, it is good to get the mare used to washing and gentle handling of these hind end areas.

Scott says, "A mare should be foaled out in a location that has experienced personnel to monitor the mare. The foaling farm should also have a close working relationship with a veterinarian who is able to
provide a rapid response time for a foaling emergency."

In situations where the mare is in a high-risk pregnancy, it is best to have her in a location where she will foal under constant veterinary observation and monitoring.

Take-Home Message

Even the most experienced mare can have complications during her pregnancy or delivery that can jeopardize her life and the life of her foal. Taking precautions, such as proper monitoring of the fetus and mare, might help you head off an impending disaster and have a healthy mare and foal.

Further Reading

See the Breeding/Reproduction category at

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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