Was My Cat the Runt? How to Tell and What It Means
By Dr. Zubair Khalid, DVM, MS, PhD ·
If your cat was the smallest kitten in the litter, you may already suspect they were the runt. There is no official label, but veterinarians generally use the term for the kitten with the lowest birth weight, especially when that weight falls well below the rest of the litter [1]. You can often tell by comparing early photos, asking the breeder or shelter for birth records, or noting whether your cat stayed noticeably smaller than littermates through the first months.
This article is educational and is not a substitute for veterinary diagnosis or treatment.
Quick Answer
- The runt is usually the kitten with the lowest birth weight in the litter, and low birth weight is a known risk factor for death in the first two months of life [1].
- Breed is the single biggest factor in birth weight, explaining about 25% of the variation between kittens [2].
- Litter size matters. Kittens born in large litters are more likely to be low birth weight and face higher mortality risk [1].
- Being male, being born in a large litter, and having a low birth weight all raise the odds of death in the first two months [1].
- A runt that survives the first few months often catches up, but some remain smaller than average and need ongoing monitoring.
How to Tell If My Cat Was the Runt
Check the birth weight if you have it
Birth weight is the most objective way to identify a runt. In a large study of purebred kittens, breed explained about 25% of the variation in birth weight, while litter size, season, the queen's age, the presence of a stillborn kitten, and the kitten's sex each explained less than 3% [2]. That means a Devon Rex runt and a domestic shorthair runt may look very different at birth, so a number alone is not enough. You need the number compared with littermates and with the breed.
Kittens born in litters with at least one stillborn tend to weigh less, and male kittens tend to weigh more than females [2]. So a small female in a litter that lost a kitten is not automatically a runt in the medical sense, but she is still at higher risk.
Look at the litter size and the queen's history
Large litters stretch the queen's resources. Kittens born in large litters are more likely to be low birth weight and to die in the first two months [1]. If your cat came from a litter of six or more, and your cat was the smallest, the odds are good that they were the runt.
The queen's age also plays a role. Birth weight increases with the age of the queen [2]. A first-time or very young mother is more likely to have smaller kittens, which can make a runt more likely.
Ask the breeder or shelter for records
Breeders who track weights often note which kitten was smallest at birth and whether they needed supplemental feeding. Shelters rarely have birth weights, but they may know the litter size and whether one kitten was held back for extra care. If you adopted a kitten alone, you may never know for certain. That is fine. What matters is what you do next.
Compare growth over the first weeks
A runt often stays behind littermates at the two-week and four-week weigh-ins. If you have a kitten now, weigh them daily on a kitchen scale that reads in grams. A kitten who is not gaining steadily, or who loses weight, needs a veterinary visit. Small kitten development depends on steady weight gain, warmth, and frequent feeding.
Runt Kitten Health Risks and What to Watch
Higher mortality in the first two months
Low birth weight is a major determinant of neonatal survival across species, and cats are no exception. In one large study, 19.7% of kittens were classified as low birth weight and 1.9% as very low birth weight, and these groups had higher mortality in the first two months [1]. That is the single most important fact about runts. The first eight weeks are the danger zone.
Hypothermia, hypoglycemia, and dehydration
Runts have less body fat and less energy reserves. They chill faster, drop their blood sugar faster, and dehydrate faster than their littermates. If your kitten feels cool to the touch, is limp, or will not eat, that is an emergency. You can read more about early warning signs in our guide to cat dehydration symptoms.
Birth defects and dystocia
Some runts are small because of a congenital problem, not just because they were crowded in the womb. A case report described multiple severe malformations in a litter of feline fetuses after the queen was exposed to a synthetic progestin used to suppress estrus [3]. That is a rare scenario, but it shows that birth weight and survival can be affected by things beyond litter size. Dystocia, or difficult birth, is also more common in purebred and brachycephalic queens, with purebreds having 2.53 times the odds of dystocia compared with crossbreds [4]. A difficult birth can leave a kitten oxygen-deprived and smaller.
Long-term size and health
Many runts catch up by six months. Others stay smaller than average for life. There is no strong evidence that being a runt causes specific adult diseases, but a cat who was underweight at birth may have a weaker start. Keep them at a healthy weight, keep vaccinations current, and watch for dental disease and joint issues as they age. If your cat shows signs of stress during vet visits or at home, see our article on cat stress signs.
Runt Kitten Care Schedule and Checklist
| Age | What to do | Why it matters |
|---|---|---|
| Birth to 2 weeks | Weigh daily in grams, keep warm, feed every 2 to 3 hours if orphaned | Runts lose heat and glucose fast |
| 2 to 4 weeks | Weigh daily, start gentle handling, watch for eyes opening | Early weight gain predicts survival |
| 4 to 8 weeks | Weigh every other day, start weaning, first vet visit | Mortality risk is highest in the first two months [1] |
| 8 to 12 weeks | Vaccines, deworming, weight check | Catch growth problems early |
| 3 to 6 months | Monthly weight and body condition check | Confirm catch-up growth |
| Adult | Annual exam, dental check, weight monitoring | Runts may stay smaller and need tailored care |
Signs That Something Is Wrong
Call your veterinarian if you see any of these in a kitten or young cat:
- Weight loss or no weight gain for 24 hours
- Refusing to eat or unable to suckle
- Feeling cold to the touch, especially the paws and belly
- Diarrhea or vomiting that lasts more than a few hours
- Labored breathing, coughing, or choking. If your cat is choking, see how to know if your cat's choking
- Sudden limping or reluctance to walk. A cat broken leg needs same-day care
- Skin wounds that ooze or smell. A cat hot spot can worsen quickly
- Lethargy, hiding, or crying that is not normal for your cat
Common Mistakes Owners Make
Assuming a small cat is automatically a runt
Some cats are simply small because of their breed or genetics. A healthy adult domestic shorthair who weighs 7 pounds may be perfectly normal. The runt label comes from birth weight and early growth, not adult size alone.
Skipping the first vet visit
Runts need a baseline exam. Your veterinarian can check for heart murmurs, hernias, and other congenital issues that may not be obvious at home. They can also set a feeding plan if your kitten is underweight.
Overfeeding to catch up
Do not force a runt to eat more than they can handle. Overfeeding can cause diarrhea and aspiration. Feed small amounts often, and let your veterinarian calculate the right amount for your kitten's weight and age.
Ignoring the queen's health
If you are breeding, the queen's age, breed, and history of dystocia matter. Purebred queens have 2.53 times the odds of dystocia compared with crossbreds, and brachycephalic queens have 3.27 times the odds [4]. Talk to your veterinarian before breeding, especially if the queen is brachycephalic or has had a difficult birth before.
Supplies and Setup That Matter for Health
- A kitchen scale that reads in grams. Weigh your kitten at the same time each day.
- A heating pad set on low, wrapped in a towel, for kittens under four weeks.
- Kitten milk replacer if the queen cannot nurse. Never use cow's milk.
- A syringe or bottle designed for kittens.
- A quiet, draft-free box with low sides so the kitten can crawl in and out as they grow.
- A notebook or phone app to track weight, feeding times, and urine and stool output.
Limitations and When to Contact a Veterinarian
No article can tell you whether your specific cat was a runt or what their long-term health will be. That requires a veterinarian who can examine your cat, review any records, and track growth over time.
Contact your veterinarian the same day if your kitten or cat:
- Has not eaten in 12 to 24 hours
- Is vomiting or has diarrhea more than twice
- Feels cool or limp
- Is breathing fast or with effort
- Has a temperature you cannot measure or that feels abnormal
Go to an emergency clinic now if your cat:
- Is choking, gasping, or has blue gums
- Cannot stand or walk
- Has a seizure
- Has a suspected broken bone or open wound
- Is unresponsive or very weak
Schedule a routine visit if your cat was a suspected runt and has not had a full exam. Your veterinarian can check weight, body condition, heart, lungs, and teeth, and set a monitoring plan.
Frequently Asked Questions
How can I tell if my kitten is a runt?
The most reliable sign is a birth weight well below the rest of the litter, especially if the kitten stays behind in the first weeks. If you do not have birth weights, look at litter size, whether the kitten needed supplemental feeding, and whether they were the last to open their eyes or start walking. A veterinarian can assess growth at a checkup.
Can a runt cat live a normal life?
Yes, many runts survive and live normal lives, especially with early feeding support and veterinary care. The highest risk is in the first two months, when low birth weight raises the chance of death. After that window, most runts catch up or stay a bit smaller but healthy.
Do runt kittens have more health problems as adults?
There is no strong evidence that runts develop specific adult diseases because of their birth size. Some may stay smaller, and any cat with a rough start deserves regular exams and weight monitoring. Your veterinarian can tailor a plan based on your cat's history.
Is the runt always the smallest cat in the litter?
Usually, yes, but not always. A kitten can be small for other reasons, such as a congenital defect or an infection. The runt label is about birth weight and early growth, not just adult size. If a kitten is failing to thrive, see a veterinarian.
Should I breed a queen who has had a runt before?
That is a decision for you and your veterinarian. Dystocia risk is higher in purebred and brachycephalic queens, and difficult births can produce smaller, weaker kittens [4]. Your veterinarian can review the queen's history and help you decide whether breeding again is safe.
Related Articles
- Cat Hot Spot or Something Else? How to Tell and What to Do
- Cat Dehydration Symptoms: How to Tell and What to Do
- How To Know If Your Cats Choking
- Cat Broken Leg: How to Tell and What to Do
- Cat Stress Signs: How to Recognize and Reduce Stress in Your Cat
- How Often Do Cats Go Into Heat? Feline Estrous Cycle & Signs Guide
- How to Tell If Something Is Wrong With Your Kitten
- How to Tell If Your Cat Is Sick
- Will My Cat Get Along With Another Cat? How to Tell
References
- Association between Birth Weight and Mortality over the Two First Months after Birth in Feline Species: Definition of Breed-Specific Thresholds. Animals : an open access journal from MDPI, 2023.
- Birth weight in the feline species: Description and factors of variation in a large population of purebred kittens. Theriogenology, 2022.
- Multiple congenital malformations in a litter of feline fetuses associated with gestational exposure to a synthetic estrus-suppressing progestin. BMC veterinary research, 2026.
- Dystocia in cats under UK primary emergency veterinary care: epidemiology, clinical management and outcomes. The Journal of small animal practice, 2026.