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Baby Size Dog Breed Calculator

Find the toy or small dog breed whose standard adult weight is closest to a typical baby's weight in late pregnancy, weeks 29 to 40.

Level: Grades 4–5

Printable worksheet

Baby Size Dog Breed Calculator

Inputs
Start from

Completed weeks since the first day of your last period, from 10 to 40.

Result

Calculating…

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

Please note: This calculator gives a general screening estimate, not a diagnosis. Talk to a qualified healthcare professional about your individual health.

What this calculator does

See which small dog breed weighs about the same as a typical baby in late pregnancy. Enter your week, or let the calculator work it out from your last period or due date. Matches start at week 29, when a typical baby first comes close to a Chihuahua's weight.

The baby's weight is a typical average for the week, not a measurement of your baby. Babies vary a lot, and only an ultrasound by your healthcare provider measures your own baby. Before week 29, every breed here is far heavier, so the page says there's no close match.

Where it's used

Why this math matters outside the classroom:

  • Middle of a range. Students find the middle of a Maltese's 3 to 4 kg standard by adding the ends and halving: 7 ÷ 2 = 3.5 kg.
  • Pounds to grams. Dog owners convert a 5 lb Pomeranian to grams by multiplying 5 by 453.59, which gives about 2,268 g.
  • Comparing weights. Dividing 2,813 g by 2,500 g gives 1.13, so a typical 36-week baby is a little heavier than a Japanese Chin.
  • Checking a limit. Students test 1,210 ÷ 2,000 = 0.61 and see that it falls below the 0.67 limit for a fair match.

How to use it

  1. Choose how to start: the week you're in, the first day of your last period, or your due date.
  2. Enter the week (10 to 40) or the date. For the date options, "Counting from" starts as today; change it to check another day.
  3. Read the closest breed. Below it you'll see the ratio, the baby's typical weight and its typical length.
  4. Open the table to see which weeks have a breed match. From week 29 on, each week has one.

What the inputs mean

Week of pregnancy
Completed weeks since the first day of your last period. Breed matches start at week 29.
First day of last period
The first day of bleeding of your most recent period. The calculator assumes a 28-day cycle.
Estimated due date
The date your healthcare provider gave you, or one from a due date calculator.
Counting from
The day to work out the week for. It starts as today.

Formula and method

Typical weight (g) = e^(0.578 + 0.332 × W − 0.00354 × W²)
Breed weight = middle of the standard's weight range = (lightest + heaviest) ÷ 2
Ratio = baby's typical weight ÷ breed weight
Fair match when the ratio is between 0.67 and 1.5

W is the number of completed weeks of pregnancy, and e is a fixed number, about 2.718. The baby's weight comes from a formula by Hadlock and colleagues (1991). It gives the middle weight for each week, so half of babies weigh more and half weigh less.

A breed standard is the written description a kennel club uses to judge a breed. Many give an ideal weight range for adult dogs. The page uses the middle of that range, such as 4 to 6 lb for a Pomeranian, which gives 5 lb (2,268 g).

Standards differ between kennel clubs, so each breed names the one used. Breeds whose standards give only a height, or only a top weight, are left out. Any middle figure for them would be a guess.

Worked example

You are 36 weeks pregnant. Which small dog breed weighs about the same as a typical baby?

  1. Weight formula: 0.578 + 0.332 × 36 − 0.00354 × 36² = 7.9422.
  2. Undo the log: e^7.9422 = 2,813 g, about 6 lb 3.2 oz.
  3. The nearest breed is the Japanese Chin. Its UK standard gives an ideal 1.8 to 3.2 kg, so the middle is 2,500 g.
  4. Ratio: 2,813 ÷ 2,500 = 1.13, which is between 0.67 and 1.5, so it's a fair match.

Result: At 36 weeks, a typical baby weighs about 2,813 g (6 lb 3.2 oz), about 1.1 times the weight of a Japanese Chin.

Understanding the result

A ratio near 1 means the two weigh about the same. A ratio of 1.13 means the baby is 13% heavier than the dog, and 0.69 means it is 31% lighter.

The earliest match is at week 29. A typical baby then weighs 1,379 g, and the lightest breed here, the Chihuahua, is set at 2,000 g. 1,379 ÷ 2,000 = 0.69, just inside the 0.67 limit. At week 28, 1,210 ÷ 2,000 = 0.61, which is too far off.

Breed standards describe the ideal show dog, not every pet. Plenty of healthy Chihuahuas and Maltese weigh more or less than their standard. Read each match as "about this heavy".

Assumptions and limitations

  • Breed weights are the middle of the adult weight range in a published breed standard. The Chihuahua and Maltese use the FCI, an international kennel club group.
  • The Pomeranian, Brussels Griffon and English Toy Spaniel use the American Kennel Club. The Japanese Chin uses The Kennel Club (UK).
  • The week is counted from the first day of the last period, with a due date 280 days after that day. Sizes are given for whole weeks.
  • From 41 weeks to 42 weeks 6 days, the 40-week figures are shown, since the weight formula stops at 40 weeks.
  • These are averages, not a measurement or a prediction for any one baby. Twins and other multiples are often smaller.
  • Only weeks 29 to 40 have a breed match. For weeks 10 to 28, use the main Baby Size by Week page.
  • Individual dogs often weigh more or less than their breed standard.
  • It is not a medical tool. It can't detect any problem, and it doesn't replace prenatal care.

Frequently asked questions

Why are there no matches before week 29?

Even the smallest breed here weighs about 2 kg as an adult. A typical baby doesn't come within 1.5 times of that until week 29. Rather than stretch the comparison, the page says there's no close match for earlier weeks.

Why not compare with newborn puppies for the early weeks?

We looked for breed-by-breed birth weights from veterinary sources. We couldn't find any we could check at the original source, so we left puppies out rather than guess.

Why isn't the Yorkshire Terrier on the list?

Its standards give only a top weight, such as "must not exceed seven pounds". Without a lowest weight, there is no honest middle figure. The same goes for breeds whose standards give only a height.

Is my baby too big or too small if my scan shows a different weight?

This page can't tell you that. The weight here is an average, and healthy babies vary widely around it. Ultrasound weights are estimates too. Your healthcare provider looks at your scans and history together, so ask them about your baby's growth.

Are the dates I enter stored or shared?

No. The math runs in your browser, and this calculator doesn't create a shareable link with your dates.

How this calculator is checked

Method: Typical weight: Hadlock, Harrist & Martinez-Poyer, Radiology 1991;181:129–33, 50th-percentile equation (10–40 weeks), with lengths and dating as in src/lib/health/fetal-size.ts. Breed weights: middle of the adult weight range in FCI standards 218 (Chihuahua) and 065 (Maltese), AKC standards (Pomeranian, Brussels Griffon, English Toy Spaniel) and The Kennel Club (UK) standard (Japanese Chin), each cited by URL in items.ts. Matching: closest weight on a ratio scale, with a 1.5× limit for a fair match (src/lib/health/themed-baby-size.ts). The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-26. Found a problem? Tell us.

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