In a show ring, that is the size you will see, but even the red ribbon winner is likely to have a mother, siblings or cousins outside that range, mostly larger as the tiniest puppies are short-lived.

Chihuahuas vary in size from <1 pound to >20 pounds, and can be shorter than 6 inches or taller than 9 inches. They do not breed true for size, meaning puppies can grow to be much smaller than both parents, much larger than both parents, or anywhere inbetween.

All female and most male Chihuahuas that are suitable for breeding have some dwarfing genes, and some large-size genes. Mom gives a random half of her genes to each puppy, as does dad, and two halves make a whole puppy.

As you might expect, a typical puppy from mid-sized parents will inherit some of mom's dwarfing genes and some of her normal-size genes, plus some of dad's dwarfing genes and some of his normal-size genes, and turn out mid-sized, just like mom & dad. But the odd puppy will inherit all mom's dwarfing genes and all dad's dwarfing genes and turn into a tiny 1-2 pound Chihuahua, and another puppy from the same parents might inherit all mom's normal-size genes and all dad's normal-sized genes and grow to 20 pounds!

Big snow white dogs like the Samoyed and Great Pyrenees may have a full set of large-size genes, or they may be heterozygous for one of the dwarfing genes. Some "runts" are the result of these dwarfing genes doubling up when a particular puppy has inherited the dwarf variant from both parents. But for the most part, the puppies of larger breeds tend to be quite consistent in size.

For little dogs, on the other hand, variation within the breed and even within each litter tends to be more common, due to a variable presence of dwarfing genes.

The first dwarfing gene, IGF1R, we will nickname "Dopey." Dopey is going to start you off towards understanding what to expect in a typical Chihuahua litter. IGF1R heterozygotes are unexpectedly common: perhaps they are particularly attractive. They are so common that in more than 75% of Chihuahua litters, one or both parents are IGF1R heterozygotes, and this results in 2-3 distinct sizes of puppy. If both parents are IGF1R heterozygotes, then some of the puppies will inherit the dwarf variant from both parents and grow to be 20% smaller than their parents; some will inherit the large variant from both parents and get 55% larger than their parents; and some puppies will inherit one of each and be of similar size to their parents. At birth, those +55% puppies are already 25% heavier than their average-sized siblings. Often, mothers are able to whelp these puppies on their own. In combination with a different large-size gene, a small litter size of only 1-2 puppies, or a mother with a narrow pelvis, this can result in dystocia and the need for a caesarian section.

Most often in a Chihuahua litter, you will also be dealing with a couple other dwarfing genes at the same time as IGF1R, so instead of 2-3 possible shapes and sizes of puppy, you'll often have 10-20, which is more possibilities than there are puppies in a litter!

The second dwarfing gene, GHR(2), we will nickname "Grumpy," Grumpy is how you may feel if you have to deal with this gene. GHR(2) heterozygotes are less common than IGF1R heterozygotes, but can play a critical role in perhaps ~10% of Chihuahua litters. If both parents are GHR(2) heterozygotes, then some of the puppies will inherit the dwarf variant from both parents and will grow to 10% larger than their parents; some will inherit the large variant from both parents and get 110% larger than their parents; and some puppies will inherit one of each and be of similar size to their parents. If it sounds strange to you that the pups with a double dose of this gene are bigger than the ones with a single dose, it is, but that's how it works for this gene, and the pups are only 10% bigger at maturity and 5% bigger at birth, so it's not really noticeable. But those +110% puppies are already 45% heavier than the average ones at birth, and usually cause dystocia and must be delivered by caesarian section.

The third and fourth dwarfing genes, STC2 and SMAD2, we will, for no particular reason, nickname "Sneezy" and "Sleepy." As always, the greatest variation in size is going to occur if both parents are heterozygous. A pair of STC2 heterozygotes would have puppies 40% smaller up to 20% heavier than themselves; a pair of SMAD2 heterozygotes would have puppies 45% smaller up to 20% heavier than themselves.

We could, for example, have a Chihuahua pair that is heterozygous for IGF1R and GHR(2) and STC2 and SMAD2. Let's say Mom is 4.7 lb and dad is 5.2 lb. The smallest puppy from them could be 1.20 x 1.00 x 1.40 x 1.45 = 2.4x smaller than themselves, i.e., 2.2 lb; and the largest 1.55 x 2.10 x 1.20 x 1.20 = 4.7x as heavy as them, i.e., 24 lb! While statistically unlikely, it is possible to have those two extremes in the same litter.

The fifth dwarfing gene, GHR(1), we will nickname "Happy", since heterozygotes are a happy medium in size. GHR(1) heterozygotes probably exist in chihuahuas. If so, then when both parents are GHR(1) heterozygotes, some of the puppies will inherit the dwarf variant from both parents and grow to be 25% smaller than their parents; some will inherit the large variant from both parents and get 30% larger than their parents; and some puppies will inherit one of each and be of similar size to their parents.

The sixth dwarfing gene, HMGA2, we will nickname "Doc", since it seems to be a rather decisive gene. Most breeds <6 kg (13 lb) have only the HMGA2 dwarfing variant; while most breeds >6 kg (13 lb) have only the HMGA2 large size variant. Overall in a multi-breed study, HMGA2 heterozygotes are unexpectedly uncommon: perhaps they are unattractive? If there are any HMGA2 Chihuahua heterozygotes, then puppies from a pair of them would range 10% smaller to 60% larger than their parents, and this gene would be a significant factor in whether the whelping of that litter would require assistance.

The seventh dwarfing gene, IGF1, we will nickname "Bashful", since it hides in large breeds the way GHR(2) hides in tiny breeds. It is unlikely that any IGF1 heterozygotes exist among Chihuahuas and most other tiny breeds, but it would be unfair to neglect this last of the 7 key dwarfing genes. A pair of IGF1 heterozygotes would have puppies 40% smaller, 10% heavier, and similar to themselves. Among large breeds, IGF1 is a significant cause of "runts."

All dogs share a same physiology - the way their organs function - and that physiology only works within a certain size range. When you push to the extremes, certain functions fail. Temperamental issues related to tiny size include extreme sensitivity and reactivity, with inadequate ability to relax or ignore neutral stimuli. Metabolic issues include a tendency towards hypoglycemia, extreme sensitivity to cold or heat, and a strain caused by the need to consume a higher amount of calories per unit body weight. Other physiological problems include heart defects and failure or deformity of other major organs. Expected lifespans of the tiniest Chihuahuas is as little as a few hours to a couple years. The smaller the Chihuahua, the more issues you are likely to see, and the more serious those issues are likely to be.

When you get up to the size where a well-bred and properly-raised Chihuahua can be a happy and healthy pet, the smallest of these are still, if female, unsuitable for breeding.

Added birth weight of the entire litter is very roughly half of maternal weight gain. In the 9 weeks from ovulation to paturition, a large dog will gain ~20% of her own body weight; whereas, in the same 9 weeks a tiny dog will gain ~40% of her own body weight, or occasionally even double in weight.

The adult size of a Chihuahua puppy is very hard to predict.

• Knowing the size of the parents will give a rough guess before the puppies are even born, but there are exceptions, e.g., All 10 of Peekaboo & Pesto's puppies together matured smaller than both parents; all 10 of Lorien's puppies matured larger than both parents.
• Birth weight and birth weight relative to littermates are moderately useful. The mother's individual physiology and number of puppies in a litter can also influence birth weight. Certain shapes of puppies tend to have a lower birth weight relative to their adult size, e.g., Indi was smallest of 5 at birth, but 2nd biggest as an adult.
• During whelping, puppy skulls change shape temporarily, but after the first couple days, they are also an indication of size. A (relatively dolichocephalic) puppy with a proportionately large square head will grow more than a (relatively brachycephalic) puppy with a small narrower head and higher forehead.
• At 8 weeks, puppy size has still been much-affected by the mother's physiology, litter size, and environment. It is somewhat useful for prediction, but there are exceptions, e.g. at 8 weeks, Zephyr was 2.2 lb, but at 8 weeks her daughter Sienna was 3.2 lb. Sienna didn't grow 50% larger, she didn't even grow 1 lb larger than her mother. As adults, Zephyr was 6.0 lb and Sienna is 6.5 lb.
• Although smaller Chihuahuas do tend to quit growing at a younger age, there is great variation among individuals; e.g., 5-6½ lb Chihuahuas may reach their adult weight as young as 5 months or as old as 13 months.

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The smallest Chihuahuas tend to have very high foreheads and round skulls, proportionately large heads, large eyes, small snippy muzzles, toothpick legs, short toes and short tails.
The largest Chihuahuas tend to have undefined stops, long muzzles and skulls, long legs, long toes, and long tails.
The middle sizes show the greatest variation, some of which you may find more attractive than others. Given 4 dwarfing genes, there would be 19 different shapes at the 5-6½ lb size alone, 16 different shapes at the 4-5 lb size, 10 different shapes at the 3-4 lb size, 4 different shapes at the 2½-3 lb size, up to 50 shapes that can fall within the 2-6 lb breed standard, and if all Chihuahuas 1-20 lb are counted, a total of 81 different shapes!

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