You weighed your puppy, checked it against a chart, and the number sits well under the line.
The most likely answer is the dullest one. Your puppy is going to be a smaller adult than average.
That is worth saying first, because the internet is not good at this question. Real medical causes of poor growth exist, and some are urgent. But “small” and “not growing” are different things. That is the distinction that matters, and you can make it at home.
Small and not growing are different things
The published guidance is clearer than most owners realise. The WALTHAM growth chart guide plots nine centile lines. A centile is a rank: a puppy on the 9th centile is lighter than about nine puppies in ten its age.
The guide says that “any centile line (or space between two centile lines) can be considered a healthy weight for an individual puppy.” Everything from the 0.4th to the 99.6th centile counts as normal.
So a puppy on the 9th centile is not a puppy with a problem. It is a small dog.
What the same guide flags is a change in pattern. It calls “any sustained drop or rise through two or more centile spaces (crossing two centile lines)” unusual, and says it “should be carefully assessed.” The same goes for a flat growth line, or worrying signs at any weight.
That threshold was tested, not asserted. Salt and colleagues (2020) found centile crossing “uncommon in healthy dogs.” Fewer than 5% crossed more than two lines. Meanwhile 49% of dogs that became underweight had gained more slowly than expected.
So the useful question is not “is my puppy below average?” It is “is my puppy still climbing along its own line?”
About half of it is simply inherited
Strandberg and colleagues (2023) put the heritability of bodyweight at about 51%. Heritability is the share of the size gap between dogs that traces back to genes. That average covers 19 breeds, and it ran from 35% to 70%. So roughly half of why your puppy differs in size from its littermate was settled at conception.
Sex takes a chunk of the rest. Andersson and colleagues (2023) looked at 114,568 dogs across 72 breeds. Three quarters of the breeds showed a clear sex difference, “with male dogs being more heavy in all breeds.” If you are comparing a female against a figure built from males, some of your gap is just that.
One honest gap. We went looking for a published figure for how much adult weight varies within a breed: the number that would let you say “80% of Labradors fall between X and Y.” We could not find one in the papers we could reach. Papers report breed means with standard errors, which is a different and much narrower thing. If you have seen a within-breed range quoted confidently, be suspicious of where it came from.
Sometimes the average is wrong, not the puppy
Three reasons the comparison itself may not hold.
Breed weight ranges are poor predictors. Smith and colleagues (2018) scored 140 purebred dogs. Their conclusion: “published recommended, breed-specific body weight ranges are not a good predictor for an ideal BCS.” BCS is body condition score, the hands-on check of whether a dog carries the right amount of fat.
Your puppy may not be the breed you were told. Gunter and colleagues (2018) DNA-tested 384 shelter dogs. Staff named the main breed correctly in only 56.7% of cases. A third of the visual guesses matched no breed the DNA found, and the dogs averaged three breeds each. It is better for owners of single-breed dogs: Sexton and colleagues (2025) found the owner’s answer matched the DNA in 86% of them, though accuracy was notably lower for dogs from shelters and rescues.
The chart may not have been built for your dog. Salt and colleagues (2017) produced the best-tested growth standards in veterinary medicine. They state that “mixed breed dogs, and those from all other breeds, were excluded,” and that the standards “do not apply dogs with an adult weight >40 kg.” If either fits your dog, how to estimate adult size when your puppy is a mix sets out the methods that do.
The runt question, answered honestly
We could not answer it, and neither can anyone else yet.
What is well established is that low birth weight predicts death in the first weeks. Mugnier and colleagues (2020) followed 4,971 puppies. Death rates came out at 4.2% for normal birth weight, 8.8% for low, and 55.3% for very low. A 2023 analysis put it at 24.6% for low-birth-weight puppies against 6.1% for normal.
But every one of those studies stops at survival. One paper followed birth weight into adulthood. Leclerc and colleagues (2017) measured adult body make-up rather than size, reporting that “in contrast to human studies, BW at birth did not correlate to the overweight status, FFM at adulthood.” FFM is fat-free mass, muscle and bone rather than fat.
So nobody has followed low-birth-weight survivors to adult size in dogs. The death-rate research is strong, but do not let it be read as evidence that a surviving runt stays small. Those are two different questions, and only one has been studied.
The medical causes worth knowing
If your puppy is failing to grow rather than simply small, these are the conditions a vet will think about. One observation you can make yourself is unusually useful. Is the puppy small all over, or are its limbs short for its body?
Intestinal parasites: the most common and the most treatable. Roundworm produces “lack of growth and loss of condition,” a dull coat, a pot belly and mucoid diarrhoea, per the Merck Veterinary Manual. Mucoid means slimy, and the worms can already be in the gut “as early as 1 week after birth.” Hookworm is the more urgent one: a “characteristic, and often fatal” anaemia can strike “as early as 1–2 weeks,” before any eggs show up in the faeces. Both pass from the mother.
Congenital portosystemic shunt: the classic cause of a stunted puppy. Congenital means present from birth. A shunt is a blood vessel carrying blood past the liver, so the blood never gets cleaned. Konstantinidis and colleagues (2023) report that most affected animals are “diagnosed at less than 1 to 2 years old,” with “poor growth or lower body condition score” among the signs. Extrahepatic shunts, outside the liver, occur in “small purebred dogs such as Yorkshire terriers, Maltese, Pugs, and Miniature Schnauzers.” Intrahepatic ones, inside it, occur in “large breed dogs, such as Labrador and Golden Retrievers, German Shepherd Dogs, Irish Wolfhounds.” One correction worth carrying: signs that worsen after a meal do point this way, but only 30–50% of patients show it, and its absence rules nothing out.
Congenital hypothyroidism: an underactive thyroid gland from birth. It produces dwarfism that is out of proportion, with limbs short for the body, and it is time-critical. Greco and colleagues (1991) described five Giant Schnauzer puppies with dwarfism, low energy, late-erupting teeth and “epiphyseal dysgenesis” (badly formed bone ends). Treatment “prior to 4 months of age” cleared it completely in four of them. The fifth “failed to attain normal breed standards for height.” Merck adds shortened backbones and growth plates that close late.
Pituitary dwarfism: too little growth hormone from the pituitary, a gland at the base of the brain. It produces dwarfism that is in proportion, and shows in a telling way. Per MSD, affected pups “are indistinguishable from unaffected littermates up to 2 months old.” After that they fall behind, with “retention of puppy coat, and lack of primary guard hairs” and slow, even hair loss. Most common in German Shepherds.
Congenital kidney disease. Merck’s renal anomalies chapter covers renal dysplasia, kidneys that never formed: “dwarfing may be noticed if the onset of renal failure occurs within the first few months after birth.” Heavy drinking and urinating usually come first. Uraemia, waste building up in the blood, is picked up between 6 months and 2 years.
Short legs that are normal for the breed. Not every odd proportion is disease. Bannasch and colleagues (2022) showed that FGF4 retrogenes (extra copies of a gene, reinserted into the genome) cut shoulder height and forelimb length. They note that “chondrodysplasia… is required by many breed standards.” Chondrodysplasia is that short-legged build. Dachshunds are nearly homozygous for both retrogenes, two matching copies of each. Beagles carry one. Short-legged here is the breed working as designed.
Two things we will not tell you, because the sources do not support them. Exocrine pancreatic insufficiency (a pancreas that stops making digestive enzymes) is described in the veterinary references as a young-adult disease, mainly in German Shepherds, not a typical cause of a small eight-week-old. And congenital heart disease is not a common explanation for a small puppy. The common heart shunts show up as a murmur in a dog that is growing fine.
When to seek advice promptly
- No weight gain week on week, or a flat growth line.
- Weight loss at any point during growth.
- Pale gums, weakness or dark tarry stools, especially in a very young puppy.
- Lasting diarrhoea or vomiting, or a dull coat with a pot belly.
- Dullness, wobbliness or seizures, above all after meals.
- Heavy drinking and urinating.
- Limbs that look short for the body, late teeth, a thickened tongue.
- A lasting drop across two or more centile lines.
What will not make your puppy bigger
Feeding more. Adult size is mostly genetic. As Larsen’s review (2010) puts it, overfeeding produces “a larger but less dense skeletal structure supporting a load that is heavier than ideal.” You cannot feed a small puppy up to average. You can give it worse joints while trying. See rapid growth and joint problems for what that costs.
Paw size will not tell you either. We looked for evidence and found none in either direction. Not a debunking: an absence of any published test, which we set out in do big paws mean a big dog?.
What to do this week
- Weigh your puppy and write it down, then repeat weekly. One measurement is a dot; four is a line, and the line is the information.
- Check body condition by hand, not by eye. Our guide covers the method.
- Look at proportions. Small all over, or short in the leg?
- Confirm the worming history with your breeder or vet. It is the cheapest thing here and the most often missed.
- Take your weight log to your next appointment. A trend answers questions a single number cannot.
If you want an estimate of where your puppy is actually heading, our puppy size calculator works from its current weight and age rather than the breed label, and reports a range instead of a single number.