One of the first questions every expectant parent asks at a prenatal visit is "how big is my baby right now?" Doctors answer in millimeters or grams from an ultrasound, but that number is hard to picture. This baby size calculator translates gestational week into an estimated length in centimeters and inches, a plain-English fruit comparison, your current trimester, and the days left until your estimated due date. It is built on the same fetal growth anchors used by the World Health Organization growth standards and the fruit-comparison charts published by major pregnancy education sites.
Type in your current week and the result card updates instantly. If you brought home an ultrasound report with a measured crown-rump or crown-heel length, plug that number into the optional field and the calculator shows how your baby's measurement compares to the average for that week — expressed as a percentage above or below the mean.
How to Use
- Enter your gestational week in the first field. This is the week count doctors use, measured from the first day of your last menstrual period — not from the date you conceived. If you are between weeks, type the number you were given at your last prenatal visit.
- Pick your preferred length unit — centimeters if you live outside the US, inches if you read ultrasound reports in imperial units. The result card shows both anyway, so this choice only controls which number appears in bold.
- Optionally enter your ultrasound length in centimeters. If you do not have a recent measurement, leave this at 0 and the calculator simply skips the comparison.
- Read the result card: the estimated length in cm and inches, the fruit your baby is roughly the size of right now, your trimester, and the days until week 40.
- If you entered an ultrasound number, the last line shows how far that measurement sits above or below the average for your week. A small difference — say within about 10% — is completely normal; growth follows a curve, not a ruler.
The fruit comparison is intentionally approximate. A lime at 12 weeks, a banana at 20 weeks, and a cantaloupe at 36 weeks are the textbook anchors; between those points the calculator interpolates the length but keeps the fruit label of the stage you are currently in.
Formula We Use
After 16 weeks the calculator uses the standard second-trimester crown-heel estimate, length in cm ≈ 1.25 × gestational weeks. Check it: 20 weeks → 1.25 × 20 = 25 cm; 30 weeks → 1.25 × 30 = 37.5 cm; 40 weeks → 1.25 × 40 = 50 cm. That single line is the mid-pregnancy reference obstetricians mentally check against. Weeks 1–15 use a short crown-rump-length (CRL) ramp anchored to the early WHO points so the number stays tiny and positive — never a negative "length" — while the baby is still being measured curled up. Results are rounded to one decimal place, and the inch figure is cm ÷ 2.54, also rounded to one decimal.
The length curve is anchored to well-known reference points published in WHO and ACOG fetal growth materials. At 8 weeks the embryo is roughly 1.6 cm long, about the size of a kidney bean. By 12 weeks — the end of the first trimester — the fetus measures about 6 cm, the size of a lime (this is CRL). At 16 weeks it is around 20 cm, comparable to an avocado as the measurement switches toward crown-heel length. At the 20-week anatomy scan, crown-heel length is about 25 cm, roughly a banana. By 24 weeks the length is close to 30 cm, an ear of corn. At 28 weeks the third trimester begins at about 35 cm, the length of an eggplant. At 32 weeks the baby is around 40 cm, close to a butternut squash. At 36 weeks — when many babies are considered "late preterm" — the length is about 45 cm, the size of a cantaloupe. By 40 weeks, a typical newborn is around 50 cm long, comparable to a small watermelon.
Between these anchors the calculator draws a straight line. Real fetal growth is not perfectly linear — the trunk and limbs accelerate at different stages — but for a quick "how big right now" sanity check, linear interpolation stays within a few millimeters of the smoothed WHO percentile curves, which is more than accurate enough for a home reference tool.
Examples
Example 1 — Twelve weeks, first-trimester scan. A parent enters week 12 with no ultrasound measurement. The calculator returns an estimated length of 6.0 cm (about 2.4 inches), labels the baby "about the size of a lime," places the pregnancy in trimester 1, and shows 196 days until the due date. This is the classic moment most parents see their baby for the first time on a screen and leave the visit saying "it's the size of a lime."
Example 2 — Twenty weeks, anatomy scan. At week 20, the average baby measures about 25.0 cm (9.8 inches), "about the size of a banana." Trimester flips to 2, and 140 days remain until week 40. If the ultrasound report says the baby measures 26.0 cm, the optional comparison field shows that measurement is about 4% above the average — completely within the normal band, which typically spans roughly plus or minus 15% around the mean at this stage.
Example 3 — Thirty-six weeks, near the finish line. At week 36 the estimated length is 45.0 cm (17.7 inches), "about the size of a cantaloupe," with only 28 days until the due date. Trimester 3. This lines up with 1.25 × 36 = 45 cm. Babies born at 36 weeks are usually breathing on their own but are still counted as late preterm, which is why obstetricians try to hold labor off until at least 39 weeks when possible.
Example 4 — Large-week sanity check. If you accidentally type 1,000,000 weeks, the calculator rejects the input rather than returning a nonsense number: gestational week must be between 1 and 42. The same guard applies to negative weeks — typing -5 returns the same error instead of a negative-length baby.
FAQ
How big is a baby at 12 weeks?
At 12 weeks the fetus is typically about 6 cm long (around 2.4 inches) and weighs roughly 14 grams. In fruit-comparison terms, that is about the size of a lime. The external genitalia have formed, the fingers and toes are separated, and most of the critical organ development is finished — which is why the risk of miscarriage drops sharply after the first trimester.
How big is a baby at 20 weeks?
At 20 weeks the average crown-heel length is about 25 cm (9.8 inches), roughly the size of a banana. This is the week the anatomy scan usually happens: the sonographer measures the head circumference, abdomen, femur and other landmarks to confirm growth is on track and check organs visually. You may also start feeling quickening around now, especially with a second or later pregnancy.
How big is a baby at 36 weeks?
At 36 weeks the baby is around 45 cm long (17.7 inches) and weighs roughly 2.6–2.9 kg, about the size of a cantaloupe. Most babies have settled into a head-down position by now. Length gains slow down over the last few weeks while weight continues to climb — the lungs and brain are the main finish-line projects, not the height.
What is the difference between crown-rump and crown-heel length?
Early in pregnancy (roughly 8–20 weeks) the legs are tucked, so sonographers measure crown-rump length (CRL) from the top of the head to the bottom of the buttocks. After about 20 weeks the baby uncurles and the standard switches to crown-heel length (CHL), measured from head to heel. This calculator uses the smoothed combined curve so you can type any week from 1 to 42 and still get a sensible length, but remember the early numbers are CRL and the later numbers are CHL.
Why does the calculator compare the baby to fruit?
Fruit comparisons are a communication shortcut. Saying "6 cm" is a number; saying "about the size of a lime" is a picture parents can hold in their hand. The NHS, BabyCenter, What to Expect and most pregnancy apps have used fruit comparisons for years because they translate abstract millimeters into something tangible. The fruit labels on this page follow that tradition and line up with the standard weekly anchors.
Is it normal if my ultrasound measurement differs from the average?
Yes. The "average" is the 50th percentile, and perfectly healthy babies sit anywhere from roughly the 5th to the 95th percentile. A measurement that is 10% above or below the curve at your week is usually nothing to worry about. Your OB or midwife tracks the trend across multiple scans, not a single number. If the difference is larger than about 15%, or if the trend keeps dropping percentile by percentile, your clinician will order a follow-up scan and Doppler studies — but the calculator itself is not the tool that diagnoses anything.
When should I stop using this calculator?
After week 40 the calculator clamps to the 40-week anchor (about 50 cm, small watermelon) because post-term growth is highly variable and is managed by your obstetrician, not by a length estimate. If you go past your due date, your care team will monitor amniotic fluid, placental function and fetal heart rate directly.
Disclaimer
This baby size calculator is provided for informational purposes only and is not medical advice. Estimated lengths and fruit comparisons are smoothed averages anchored to published growth charts; every pregnancy and every baby is different, and actual measurements from your own ultrasound are the only numbers your obstetric care team should use for clinical decisions. Always confirm fetal growth, due dates and any concerns with your doctor, midwife or prenatal clinic. Do not delay seeking professional care because of anything this page tells you.