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Overdue Pregnancy

Overdue Pregnancy
Cheryl Lopate D.V.M.

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IMG_4623I have a 14 yr quarter horse, five previous foals, typical gestation 12 months. Currently bred to a Jack for mule foal. Breeding dates May 15-June 20, 2015. Going by last breeding date, she is 401 days. Positive pregnancy test. Can see foal moving in abdomen and hind end. Producing milk but bags are not full. Milk is thick, sticky, opaque and sweet. Hind end relaxed, vulva slightly swollen. No “v” shape in abdomen, in fact, she’s not excessively large. She has been like this, excluding sweet tasting milk for six weeks. I am in remote Montana and closest vets want me to haul her into their facilities if I want her seen. All three vets called have basically said “wait and see” and that they will not do an internal ultrasound or palpate because it requires sedation and she is too far along for that. I’m about to pull my hair out. – Jennifer


Hi Jennifer,

Mares bred to donkeys (carrying mule fetuses) typically have a pregnancy length of 372 -374 days (so over a month longer than normal mare pregnancy) but mares may go well beyond their ‘expected’ due date with ‘normal’ pregnancies. They will typically foal when the fetus is mature. So a mare that goes over significantly (more than 30 days) over her due date may have some placental dysfunction resulting in slower maturation of the fetus and a delay in foaling. Parturition (foaling) should not be induced unless there is clear evidence of fetal maturation as this may result in a foal being born dysmature which increases it’s risk of postpartum complications (dummy foal, limb abnormalities, etc).

Another possible reason a mare goes overdue is incorrect calculation of when pregnancy was established. If a mare has been turned out with a male and a breeding was witnessed, the pregnancy may be believed to have been established with that mating; however, if she was left out with the male, he may have covered 21 or 42 days later as well, but the second or third breeding may not have been noted, so the assumption is that the pregnancy was established on the first breeding, when in fact it was established 1 or 2 cycles later.

A far less common reason would be some type of brain defect in the fetus where the pituitary gland doesn’t develop normally so that the normal hormonal cascade from the fetus does not occur to initiate parturition. These fetuses tend to grow to enormous sizes without initiating labor.

Monitoring calcium levels in milk can be performed using water hardness strips (those that test only for calcium, not calcium and magnesium) or by using commercial kits made for this purpose. Elevations in calcium, along with inversion of the sodium:potassium ratio indicate fetal readiness for birth. Calcium levels of 10 mmol/L (40 mg/dL; 400 ppm) indicate readiness for birth and typically foaling will follow within 24 – 48 hours of calcium levels reaching these levels.

Consulting with a veterinarian on the normalcy of the pregnancy is recommended if you have any concerns.

Cheryl Lopate, MS, DVM
Diplomate, American College of Theriogenologists
Board certified in Animal Reproduction

Reproductive Revolutions
18858 Case Rd NE  (equine facility)
Aurora, OR 97002
503-982-5718 (fax)

Wilsonville Veterinary Clinic (small animal facility)
9275 SW Barber St
Wilsonville, OR 97070
503-682-3540 (fax)

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Ask The Experts
Cheryl Lopate D.V.M.

Dr. Lopate received a BS in Animal Science from Colorado State University in 1984 and a Masters Degree in Reproductive Physiology in 1987 from The Ohio State University. She graduated from The Ohio State University College of Veterinary Medicine in 1991. She practiced in a rural mixed practice in Minnesota from 1991 to 1995, then went on to complete a residency in comparative theriogenology (reproduction) at Purdue University from 1995 to 1997 and was board certified in Theriogenology in 1997. She continued to teach clinical reproduction at Purdue University Veterinary Hospital for 2.5 more years, and then returned to clinical practice, providing primarily reproductive care for horses and companion animals until she opened her own reproduction specialty practice in November of 2003.

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