Pregnancy tool

Baby Size by Week — Fetal Development Tracker

Slide through every week of pregnancy from 4 to 40. See how big your baby is — compared to a fruit, in centimetres and inches, in grams and ounces — and exactly what's developing.

Last reviewed 20 May 2026

Week

12

Week 4Week 20Week 40
First trimesterWeek 12 of 40

At week 12, your baby is about the size of a

Lime

Length

5.4 cm · 2.13 in

Crown to rump (legs curled)

Weight

14 g · 0.49 oz

Under 1 lb

End of the first trimester — reflexes develop and risk of miscarriage drops.

What's developing this week

  • Reflexes develop — the fetus can curl fingers and toes.
  • The digestive system begins practising contractions.
  • Many providers can hear the heartbeat with a Doppler.
Find your week with the Due Date Calculator

How to use this tracker

  1. Drag the slider — or tap Prev / Next — to move between weeks 4 and 40.
  2. Each week shows a size comparison, length, weight, and a list of what's developing.
  3. The page URL updates with your selected week, so you can bookmark or share a specific week (for example, ?week=20).
  4. Use "Print this card" to save a clean copy of any week.
  5. Not sure which week you're in? Run the Due Date Calculator first.

Background: how fetal growth is measured

Crown-rump vs. crown-heel length

Up to week 19, fetal length is measured as crown-rump length (CRL) — from the top of the head to the bottom of the buttocks, with the legs curled up. This is the most reliable early measurement and is also what dating ultrasounds use. From week 20 onward, the legs are extended enough to measure, so length switches to crown-heel length (head to toe). That's why the length figure appears to jump between week 19 and week 20 — it's a change in what is being measured, not a sudden growth spurt.

Why your baby may differ from the numbers

The figures here are 50th-percentile medians. Healthy babies span a wide range around the median. A baby measuring at the 30th or 70th percentile is completely normal. Clinicians watch the trend of growth across scans, and whether a baby is tracking consistently along its own curve — not whether it matches a single median number on any given week.

Trimester milestones at a glance

  • Weeks 4–13 (first trimester): all major organs and body systems form. This is the most sensitive window for development.
  • Weeks 14–27 (second trimester): rapid growth, the anatomy scan (~week 20), first movements felt, and the viability threshold around week 24.
  • Weeks 28–40 (third trimester): dramatic weight gain, brain maturation, lung development, and final preparation for birth.

How to interpret what you see

Treat this tracker as an educational map, not a diagnostic tool. The "what's developing this week" lists describe typical development — the actual timing of any individual milestone varies. Use it to understand the broad arc of pregnancy and to know what your provider is checking for at each stage.

Key reference points worth knowing:

  • Week 12: end of the first trimester; miscarriage risk drops markedly.
  • Week 20: the halfway mark and the typical timing of the detailed anatomy scan.
  • Week 24: the generally cited threshold of viability with intensive care.
  • Week 28: third trimester begins; daily kick counting becomes useful.
  • Weeks 37–42: the full 'term' window — a normal-timed birth can happen anywhere in it.

What this tracker does NOT do

  • It does not assess your individual baby's growth — only your provider's scans can do that.
  • It does not diagnose growth restriction, macrosomia, or any condition.
  • It does not replace prenatal appointments or the anatomy scan.
  • Median figures are not targets — being above or below the median is usually normal.

Sources

  • American College of Obstetricians and Gynecologists — patient education materials on fetal development.
  • National Health Service (NHS) — week-by-week pregnancy guide.
  • Papageorghiou AT, et al. International standards for fetal growth based on serial ultrasound measurements: the Fetal Growth Longitudinal Study of the INTERGROWTH-21st Project. The Lancet. 2014.
  • World Health Organization — fetal growth charts and standards.
  • Hadlock FP, et al. Estimation of fetal weight with the use of head, body, and femur measurements. American Journal of Obstetrics & Gynecology. 1985 (re: biometry).

Our editorial process is described in our methodology. This tracker is educational and is not a substitute for medical advice — read our medical disclaimer.

Frequently asked questions

How accurate are these size and weight figures?
The length and weight values are 50th-percentile (median) averages drawn from published fetal-growth references including ACOG materials, the NHS, and WHO / INTERGROWTH-21st fetal growth standards. Every fetus grows at its own pace — your baby being a little larger or smaller than the median is completely normal. Your provider tracks growth against percentile curves at scans rather than against a single median figure.
Why does the length number jump between week 19 and week 20?
It's a measurement-convention change, not a growth spurt. Through week 19, fetal length is measured as crown-rump length (CRL) — head to bottom, with the legs curled. From week 20 onward, the legs can be measured, so length is reported as crown-heel length (head to toe). The fetus didn't suddenly get much longer; we just started measuring the legs.
What does the fruit/vegetable comparison actually mean?
It's a friendly way to picture scale — comparing the fetus to a familiar object roughly that size. It's an approximation of overall size, not a precise measurement. The exact length and weight figures on each week's card are the meaningful numbers.
When is the fetus considered 'viable'?
Around week 24 is generally regarded as the threshold of viability — the earliest point at which survival outside the womb becomes possible with neonatal intensive care. Survival rates and outcomes improve substantially with each additional week. This is a general reference point, not a guarantee; outcomes depend on many individual factors.
When will I feel my baby move?
Most people first feel fetal movement ('quickening') between weeks 16 and 22 — often around week 18-20 for a first pregnancy, and sometimes earlier in later pregnancies. From week 28, daily kick counting becomes a useful habit; our Kick Counter tool helps with that.
What's the difference between gestational age and how big my baby is?
Gestational age (the 'week' you're in) is counted from the first day of your last period. Size is a separate measurement. Two babies at the same gestational age can differ in size and both be perfectly healthy — which is why providers use percentile ranges, not single numbers.
Can I use this to track my own pregnancy week by week?
Yes — use the slider or the Prev/Next buttons to move to your current week. The URL updates as you move, so you can bookmark or share a specific week. To find your current week, use our Due Date Calculator first.
My scan measured my baby differently — should I be worried?
Not on its own. Ultrasound biometry has its own margin of error, and natural variation is wide. A measurement a week or two off the median is common and usually not concerning. Always let your provider interpret scan measurements in the full clinical context.

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