Donkeys carry their foals for about 12 months, though the actual length varies from roughly 340 to 390 days depending on the individual, the time of year she was bred, and the sex of the foal. That is noticeably longer than the roughly 11-month pregnancy typical of horses, and the wide range catches many first-time donkey owners off guard. Understanding what shapes that timeline, and how to manage a jenny through it, makes the difference between a smooth foaling and a stressful guessing game.
What the Research Actually Reports
Two large studies give the clearest picture of donkey gestation length, and their numbers differ just enough to be worth noting. A study of three Spanish donkey breeds found an average of 362 days with a standard deviation of about 15 days, meaning most pregnancies fell somewhere between 347 and 377 days.1PubMed. Factors affecting gestation length and estrus cycle characteristics in Spanish donkey breeds reared in southern Spain A study of the Martina Franca breed in Italy reported a higher average of 371 days.2PubMed. Factors affecting pregnancy length and phases of parturition in Martina Franca jennies The Martina Franca is a large draft-type donkey, and while the Spanish study found no significant breed effect among the three breeds it examined, the difference between studies hints that body size or regional genetics could nudge the average up or down.
A practical way to think about it: plan for around 12 months, but don’t panic if the jenny goes a week or two early or two to three weeks late. The spread in donkeys is wider than in most horse breeds, and that variability is normal rather than a sign of trouble.
What Pushes the Due Date Earlier or Later
Several factors influence whether a jenny delivers closer to 350 days or 380 days. The Spanish breed study found that the breeding season mattered: jennies bred early in the season tended to carry longer than those bred later.1PubMed. Factors affecting gestation length and estrus cycle characteristics in Spanish donkey breeds reared in southern Spain That pattern mirrors what happens in horses, where animals bred in winter or early spring under shorter daylight hours often have slightly extended pregnancies compared to those bred during peak summer.
The sex of the foal is another consistent influence. The Martina Franca study found that jennies carrying male foals averaged about 376 days, while those carrying females averaged 371 days.3Journal of Equine Veterinary Science. A Review of Reproductive Biology and Biotechnologies in Donkeys A separate study examining placental ultrasound measurements in donkeys confirmed that the season and the foal’s sex both affected gestation length, while other variables they measured did not.4PubMed. Transrectal ultrasonography of the caudal placental pole and fetal eyeball diameter and associations with the season, duration of gestation, placental weight, sex of the foal, and birthweight in donkeys Male foals tending to take a few extra days is a well-known pattern in horses as well, though the mechanism behind it is still debated.
The age of the jenny, the specific year, and whether the pregnancy was a first or a repeat did not significantly change gestation length in the Spanish study. That finding is useful for owners: you can’t predict the date more precisely by knowing whether your jenny is a maiden or an experienced mother.
Donkeys Versus Horses and Mules
One of the most common questions people have once they learn donkeys carry for about a year is how that compares to horses. The short answer is that donkeys are pregnant for roughly a month longer. A comparative study examining placentas at term found median gestation lengths of 371 days in donkeys versus 341 days in Thoroughbreds and 327 days in ponies.5Theriogenology. A comparative stereological study of the term placenta in the donkey, pony and Thoroughbred The difference is not just a curiosity. It has real consequences when a donkey stallion (jack) is crossed with a horse mare to produce a mule, because the mare’s reproductive system and the hybrid embryo essentially negotiate a compromise.
A study comparing mule pregnancies to normal equine pregnancies found that mule pregnancies were actually slightly shorter than horse pregnancies, averaging about 341 days versus 347 days for horses.6PubMed Central. Are mule pregnancies really longer than equine pregnancies? Comparison between mule and equine pregnancies That result surprised the researchers, because the conventional wisdom in the equine world was that mule pregnancies ran longer. The data suggested otherwise: the horse mare carrying a mule foal delivers on a timeline closer to her own species’ norm rather than stretching toward the donkey’s longer gestation.
At the other end of the spectrum, Somali wild asses in a captive breeding program at Basel Zoo recorded gestation periods of 377 to 404 days, measured from the last successful mating to birth.7ResearchGate / International Zoo Yearbook. Husbandry and breeding of Somali wild ass Equus africanus somalicus at Basel Zoo, Switzerland That range is longer still than domestic donkeys, which makes sense given that wild asses are a larger-bodied ancestor. It also illustrates how much variation exists even within the broader donkey family.
Early Pregnancy and How Vets Confirm It
Donkey pregnancy can be confirmed by transrectal ultrasound surprisingly early. One detailed study tracked early embryonic development in jennies and found that the embryonic vesicle was first visible at around 12 days after ovulation, measuring about 6.5 millimeters. The vesicle lost its spherical shape around day 18 or 19, the embryo itself became visible around day 22, and a heartbeat could be detected by about day 25.8PubMed. Clinical, ultrasonographic, and endocrinological studies on donkey pregnancy Those milestones closely mirror what happens in horse pregnancies, which means a vet experienced with horses can apply much of the same ultrasound timeline to donkeys.
Hormonal patterns in early pregnancy are also broadly similar to horses but have some donkey-specific quirks. In pregnant jennies, progesterone rises to peak levels between about days 7 and 11 after ovulation, then declines slightly before dropping more gradually through at least day 30.9PubMed Central. Early Pregnancy in Jennies in the Caribbean: Corpus Luteum Development and Progesterone Production, Uterine and Embryo Dynamics, Conceptus Growth and Maturation Progesterone levels in pregnant jennies are higher than in non-pregnant ones from the earliest days, which is what makes blood-based pregnancy testing possible. In the later months, the hormone pattern that supports the pregnancy continues to resemble the mare’s general trajectory, though some peaks are lower in jennies.8PubMed. Clinical, ultrasonographic, and endocrinological studies on donkey pregnancy
When a jenny is carrying a donkey foal sired by a jack, her hormonal profile actually looks different from a jenny carrying a mule foal sired by a stallion. Research comparing these two pregnancy types found that donkey-sired pregnancies showed lower progesterone and lower levels of equine chorionic gonadotropin compared to stallion-sired pregnancies, along with differences in blood flow to the uterus.10PubMed Central. Endocrine and uterine hemodynamic differences between intra- and interspecific equine pregnancies during early gestation For practical purposes, this means that a vet monitoring a jenny bred to a stallion may see hormone readings that differ from normal donkey-on-donkey reference ranges, and that is expected rather than alarming.
Monitoring the Placenta Through Pregnancy
In horse medicine, measuring the combined thickness of the uterus and placenta with ultrasound (a measurement vets call the “combined utero-placental thickness,” or CUPT) is a standard way to check whether the placenta is healthy. The technique carries over to donkeys, but with an important difference. In Martina Franca donkeys, researchers found that the placental thickness increases continuously from the sixth month through the twelfth month, with a substantial jump in the final three months.11Theriogenology. Transrectal ultrasonographic evaluation of combined utero-placental thickness during the last half of pregnancy in Martina Franca donkeys That pattern is different from what vets see in mares, where the increase tends to plateau earlier.
A field-based ultrasound study of pregnant jennies also found that the combined utero-placental thickness was not as reliably associated with gestational age as it is in horses, and that thickening or swelling of the membranes, which would raise red flags in a mare, was frequently observed in jennies without any associated foal problems.12PubMed. Field ultrasound evaluation of some gestational parameters in jennies The takeaway for owners and vets is that donkey pregnancy monitoring cannot simply copy horse reference values. Donkeys need their own baseline numbers, and research is still building those norms.
Feeding a Pregnant Jenny
One of the more counterintuitive findings about donkey pregnancy involves appetite. You might expect a pregnant jenny to eat more, but a study measuring daily food intake found that pregnant donkeys in late gestation actually ate about 31 percent less dry matter per kilogram of body weight compared to non-pregnant jennies on the same diet.13PubMed. Donkey Feeding During Maintenance, Pregnancy, and Lactation: Effects on Body Weight, Milk Production, and Foal Growth The growing foal takes up abdominal space, compressing the gut and physically reducing the jenny’s capacity. This means you may need to offer a more energy-dense feed in smaller, more frequent meals during the final months rather than simply offering more hay.
Donkeys are famously efficient at extracting calories from low-quality forage, and that efficiency creates a unique risk during pregnancy. Overweight jennies are prone to a condition called hyperlipemia, where the body mobilizes too much fat into the bloodstream. A survival analysis study found that female donkeys and those in obese body condition were at the highest risk for developing hyperlipemia.14Journal of the American Veterinary Medical Association. Survival analysis approach to risk factors associated with hyperlipemia in donkeys Although pregnancy and lactation are widely cited as triggers, this particular study found that being female and obese were the strongest independent risk factors regardless of pregnancy status. The practical lesson is to manage a jenny’s body condition before and during pregnancy rather than waiting until a crisis develops. An overweight jenny going off her feed in late pregnancy, even briefly, can tip into dangerous fat mobilization very quickly.
Twin Pregnancies and How They Are Managed
Twin pregnancies are a well-known risk in horses, where carrying two foals to term usually ends badly for both. Donkeys face the same issue, but some of the natural resolution differs. In a study of 25 twin pregnancies in Martina Franca donkeys, most of the unilateral twins (where both embryos implanted on the same side of the uterus) resolved on their own: about 88 percent naturally reduced to a single embryo by day 20 or 21 after ovulation. Bilateral twins, where one embryo implanted on each side, never resolved naturally.15PubMed Central. Twin Pregnancy in the Martina Franca Donkey Breed Managed by Natural Reduction and Post-Fixation Manual Crushing
When natural reduction did not occur, the remaining twins were managed by manually crushing one embryo after it had fixed in position. The combined approach achieved a 92 percent single-embryo pregnancy rate by day 28, and 87 percent of those pregnancies went on to produce a live foal.15PubMed Central. Twin Pregnancy in the Martina Franca Donkey Breed Managed by Natural Reduction and Post-Fixation Manual Crushing This is why early ultrasound checks matter. Catching twins early gives the vet the best window to intervene before both embryos are too firmly established.
Colostrum and the First Hours After Birth
Donkey foals, like horse foals, are born with almost no circulating antibodies. They rely entirely on colostrum, the first milk the jenny produces, to acquire passive immunity. The quality of donkey colostrum appears to be high. A study of Brazilian Nordestino donkeys found that the colostrum was of great quality and effectively transferred immunoglobulins to newborns.16Journal of Equine Veterinary Science. Evaluation of colostrum quality and passive immunity transfer in donkeys of the Brazilian Nordestino ecotype via different methods
How quickly the foal nurses matters enormously. A study tracking immunoglobulin (IgG) levels in donkey foals found that colostrum IgG concentrations dropped steadily over time, while foal blood IgG levels rose sharply during the first six hours after birth and continued climbing through 12 hours, with no significant further increase between 12 and 24 hours.17PubMed Central. Evaluation of jennies’ colostrum: IgG concentrations and absorption in the donkey foals. A preliminary study All foals in that study reached adequate IgG levels, but the data makes the window clear: the foal needs to be up and nursing within the first few hours. If you are attending a donkey birth, the most important thing you can do is make sure the foal latches on promptly and that the jenny is willing to stand and let it nurse. A foal that has not nursed within four to six hours after birth should be evaluated by a vet, because its ability to absorb antibodies from the gut decreases rapidly as the hours pass.
Donkey foals tend to be hardier than their reputation suggests, but they can also be deceptively stoic. A donkey foal that appears sleepy and uninterested in standing may actually be in trouble, while a horse foal in the same state would be more obviously agitated. Keeping a calm but attentive watch during the first 24 hours, making sure the foal nurses repeatedly and passes its first manure, gives the best start to a pregnancy that took a full year to reach this point.