Stillborn puppies result from a range of causes, including infections, difficult labor, hormonal imbalances, genetic defects, and environmental exposures. Reported puppy mortality rates vary between 8% and 20% depending on breed, maternal condition, and how the birth is managed. In one study of Labrador Retrievers, about 8.4% of all puppies born were stillborn, and only 57% of litters had every puppy survive. Understanding what goes wrong can help breeders reduce losses in future litters and identify problems that need veterinary attention.
Difficult Labor and Oxygen Deprivation
The single most common cause of stillbirth during delivery is prolonged labor, known as dystocia. When a dam strains for more than an hour without producing a puppy, the situation is classified as obstructive dystocia. Each unborn puppy depends entirely on the placenta and umbilical cord for oxygen. Normal contractions cause brief, harmless dips in oxygen flow, but when labor stalls, those dips become prolonged and severe. The result is progressive oxygen deprivation, a buildup of acid in the puppy’s blood, and eventually death.
Research shows that every additional hour of dystocia increases the severity of placental damage, including tissue death and congestion of blood vessels. The risk of severe placental congestion rose by 44% for each extra hour of difficult labor in one study. This is why early intervention matters so much. During a normal delivery, each puppy arrives within about 30 minutes. Gaps of up to two hours between puppies can be normal, but anything beyond that warrants a call to your veterinarian. Small and miniature breeds are especially prone to dystocia because of the size mismatch between puppy heads and the birth canal.
Bacterial and Viral Infections
Two infections are particularly notorious for causing stillbirths in dogs: brucellosis and canine herpesvirus.
Brucellosis. Caused by the bacterium Brucella canis, this infection invades the placenta and destroys the tissue that transfers oxygen and nutrients to the developing puppies. The bacteria colonize cells lining the placental villi, causing inflammation of the uterus and placenta. Most abortions from brucellosis happen between days 45 and 55 of the roughly 63-day pregnancy. When puppies aren’t aborted outright, they’re often born alive but extremely weak, with high rates of death in the first days of life. Aborted or stillborn puppies from brucellosis-infected dams often show signs of pneumonia, heart inflammation, kidney hemorrhage, and liver damage. One of the tricky aspects of this disease is that many infected dogs show no obvious symptoms beyond mildly enlarged lymph nodes, making testing before breeding essential.
Canine herpesvirus (CHV-1). This virus causes fetal death and abortion when a dam is infected during pregnancy. Puppies that survive to birth but were exposed in utero or shortly after can develop a rapidly fatal systemic infection. CHV-1 is widespread in the dog population, and dams with no prior exposure are at the highest risk during pregnancy because they lack protective antibodies.
Hormonal Imbalances
Progesterone is the hormone that sustains canine pregnancy from start to finish. Dogs are unusual among mammals in that the ovaries (specifically the corpus luteum) are the sole source of progesterone throughout gestation. There is no placental takeover as in humans. If progesterone drops too early, the uterus begins contracting and the cervix dilates, ending the pregnancy.
The critical thresholds shift as pregnancy progresses. Progesterone should stay above 20 ng/mL during the first 30 days, above 5 ng/mL from days 30 to 45, and above 1.5 ng/mL from days 45 to 58. A drop below 2 ng/mL for more than 24 hours at any point typically triggers pregnancy loss. In one study of 98 pregnant dogs, more than half had progesterone levels that fell below recommended thresholds at some point during gestation. Supplementation can help if low levels are caught in time through blood monitoring, which is why many reproductive veterinarians recommend periodic progesterone checks during pregnancy.
Genetic and Congenital Defects
Some puppies are stillborn because of structural abnormalities that make life outside the womb impossible. The most common congenital malformations in dogs include cleft palate, hydrocephalus (fluid buildup in the brain), and anasarca (severe generalized swelling). Brachycephalic breeds, those with shortened skulls like Bulldogs, Boston Terriers, and Pugs, have a higher rate of these defects.
The causes of congenital malformations are often tangled together. Genetics play a role, but so do nutritional deficiencies and environmental exposures during critical windows of development. Folate (the B vitamin also known as folic acid) is protective against neural tube defects like spina bifida, anencephaly, and hydrocephalus, all of which can lead to stillbirth. Ensuring the dam receives complete nutrition before and during pregnancy reduces this risk.
Toxic Exposures and Medications
A surprisingly long list of common medications can cause abortion or birth defects in pregnant dogs. Drugs that may trigger pregnancy loss include certain hormones, glucocorticoids (steroids), some anesthetics, and anticancer drugs. Teratogenic medications, those that cause structural defects in developing puppies, include several antibiotics, antifungals, anti-seizure drugs, and even high doses of vitamins A and D.
The timing of exposure determines the type of damage. Teratogens present during the first 26 days of pregnancy tend to cause defects in the brain, eyes, ears, and heart. Exposure during the following 26 days is more likely to affect the palate, cerebellum, and urogenital system. Many of these medications are routinely prescribed to non-pregnant dogs, which is why confirming pregnancy status before treatment is important. Common household chemicals and pesticides can also pose risks, though these are less well documented.
Maternal Health and Nutrition
The dam’s overall condition directly affects fetal survival. Obesity, being underweight, untreated infections, and chronic illness all raise the risk of stillbirth. Poor nutrition during pregnancy is a particular concern because puppies develop rapidly in the final trimester, and nutritional shortfalls during this window can cause organ defects or growth restriction that leads to death before or during birth.
Folate deficiency deserves special attention. Neural tube defects, which include spina bifida and hydrocephalus, are among the most serious birth defects and are directly linked to inadequate folate levels around the time of conception and in early pregnancy. Feeding a high-quality diet formulated for gestation and lactation, rather than a standard adult maintenance diet, is one of the simplest ways to reduce nutritional risk factors.
How the Cause Is Determined
When a litter includes stillborn puppies, identifying the cause requires more than a visual examination. A full workup ideally includes the stillborn puppies themselves, the placentas, and a blood sample from the dam. Tissues from the puppies’ lungs, liver, kidneys, and stomach contents are tested for bacterial infections including Brucella, Campylobacter, and Salmonella. If the kidneys show hemorrhaging, herpesvirus testing is warranted. The dam’s blood is checked for antibodies to herpesvirus, brucellosis, and leptospirosis.
Tissue samples are also preserved for microscopic examination, which can reveal placental damage, signs of oxygen deprivation, congenital organ defects, or inflammatory patterns that point toward infection. Collecting these samples promptly after delivery gives the best chance of a clear diagnosis. Without this information, determining whether the cause was infectious, genetic, hormonal, or related to labor complications is largely guesswork, and knowing the cause is what allows breeders to make informed decisions about future breedings.