Pregnancy · Development

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 28 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
What does this mean?
Pregnancy is dated from the first day of the last menstrual period (LMP), not from conception — that’s why “week 4” is really only ~2 weeks after the egg and sperm met. The fetus develops in three phases: germinal (weeks 0–2, implantation), embryonic (weeks 3–10, all organ systems form — the highest-risk window for teratogens), and fetal (weeks 11–40, growth and maturation). Key viability and care milestones: ~24 weeks (limit of viability with NICU care), 28 weeks (good neonatal outcomes with full intensive care), 37 weeks (early term — most lung-maturity issues resolved), 39 weeks (full term, lowest baseline neonatal risk). Length and weight charts here are population medians (Hadlock / WHO); your baby may track higher or lower and still be perfectly healthy.

How big is my baby this week?

Use the slider or the Prev / Next buttons above. Each week shows a size comparison, length, weight, and a list of what’s developing. Length and weight are 50th-percentile medians — your baby may be slightly larger or smaller and still be perfectly healthy.

Quick reference — baby size at key weeks

  • Week 8: Raspberry — 1.6 cm, < 1 g.
  • Week 12: Plum — 5.4 cm, 14 g.
  • Week 16: Avocado — 12 cm, 100 g.
  • Week 20: Banana — 16 cm, 300 g (anatomy scan).
  • Week 24: Sweetcorn — 30 cm, 600 g (viability).
  • Week 28: Aubergine — 37 cm, 1 kg (third tri starts).
  • Week 32: Coconut — 42 cm, 1.7 kg.
  • Week 36: Honeydew melon — 47 cm, 2.6 kg.
  • Week 40: Watermelon — 50 cm, 3.4 kg.

How is fetal length measured?

  • Up to week 19: crown-rump length (CRL) — top of head to bottom of buttocks, legs curled.
  • From week 20: crown-heel length — head to toe (legs extended).

Length figures jump 8-10 cm between week 19 and 20 because of this measurement change — not a sudden growth spurt. Some charts use crown-rump throughout; others switch at 20 weeks. We follow the convention used by most clinicians.

Why your baby may be different from these numbers

The figures 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 across scans, not whether any single measurement matches a median number. Ultrasound biometry itself has a margin of error of ±10% in the third trimester.

Trimester milestones — what your baby is doing

First trimester (weeks 4-13)

  • All major organs and body systems form.
  • Neural tube closes by week 6 (folic acid critical here).
  • Heartbeat from week 6.
  • Recognisable human form from week 9-10.
  • Major structures formed by week 12.
  • Most sensitive window for development — medications, infections, alcohol have biggest impact.

Second trimester (weeks 14-27)

  • Rapid growth.
  • First movements felt (quickening) typically 16-22 weeks.
  • Anatomy scan around week 20.
  • Functional hearing from week 16-18; consistent response to sound from week 25.
  • Viability threshold around week 24.
  • Eyes open around week 26.

Third trimester (weeks 28-40)

  • Dramatic weight gain — roughly triples from 1 kg to 3-3.5 kg.
  • Major brain growth (~100,000 neurons per hour from 25 weeks).
  • Lung surfactant production accelerates.
  • Brown fat deposition for temperature regulation.
  • Iron / nutrient storage for first months of life.
  • Sleep cycles emerging (sleeps ~80% of the time by term).
  • Position descends into pelvis from 36 weeks.

When does my baby start hearing me?

Ears form from week 8. Functional hearing from week 16-18. By week 25, babies respond reliably to sound — heart rate changes, movements increase. By week 28, babies recognise repeated sounds (familiar voices, music). Postnatal studies show babies prefer voices and stories they heard in utero. Talking, reading, and singing to your bump from the second trimester has gentle bonding benefits.

What does the 20-week anomaly scan actually look at?

Detailed ultrasound usually performed 18-22 weeks. Checks:

  • Brain structures and skull.
  • Face, lip, palate.
  • Heart — four chambers, outflows, rhythm.
  • Spine.
  • Stomach, kidneys, bladder, abdominal wall.
  • Arms, legs, hands, feet.
  • Placental position.
  • Amniotic fluid volume.
  • Cervical length (sometimes added).

Detects most major structural abnormalities. Doesn’t detect chromosomal conditions (those need NIPT, NT screen, or amniocentesis). Sex of baby is usually identifiable but not the purpose of the scan.

When is my baby viable?

Conventional viability is 23-24 weeks. Survival statistics improve substantially with each week:

  • 23 weeks: ~20-35% survival.
  • 24 weeks: ~50-60%.
  • 25 weeks: ~70-80%.
  • 26 weeks: ~85-90%.
  • 28 weeks: ~95%+.

UK and most developed-country NICUs offer active resuscitation from 22 weeks onwards in many centres, with parental shared decision-making at 22-24 weeks.

Practical scenarios — common worries

Scenario 1: 28-week scan says baby is 1 kg, but the median is 1.1 kg — should I worry?

No. 1 kg at 28 weeks is roughly the 25th-30th percentile — completely within normal range. The TREND matters — is baby continuing to grow along their own curve? Single readings have ±10% margin of error.

Scenario 2: 20-week anatomy scan went well but I haven’t felt movement yet

Common with anterior placenta or higher BMI. First-time mums often don’t feel movement until 20-22 weeks. If you reach 24 weeks without movement, mention to midwife for a quick listen-in.

Scenario 3: 32-week measurement says baby is in 90th percentile

Big baby. Worth ruling out gestational diabetes if not already screened. Check fundal height. Plan growth scan at 36 weeks to re-assess. Doesn’t automatically mean caesarean — many large babies deliver vaginally.

Scenario 4: 24 weeks and the bump “looks small”

Bump appearance varies hugely — depends on your body shape, baby’s position, amniotic fluid, your weight gain. As long as fundal height measurement is within 2-3 cm of expected and scans show normal growth, appearance isn’t a worry.

Scenario 5: 36 weeks and worried baby has “quietened down”

Movements should NOT reduce at term — old myth. Call your maternity unit immediately for assessment if you feel a change in your baby’s movement pattern. See /calculators/kick-counter.

Care guidance — supporting healthy fetal development

  • Folic acid 400 mcg/day from 3 months pre-conception until 12 weeks (5 mg if higher risk).
  • Mediterranean-style eating with adequate iron, calcium, vitamin D, iodine, choline, omega-3.
  • Avoid alcohol entirely.
  • Don’t smoke and limit second-hand smoke.
  • 150 min/week moderate exercise.
  • Stay hydrated 2-3 L/day.
  • Attend all antenatal appointments and scans.
  • Vaccines — influenza (any trimester), pertussis (16-32 weeks), COVID booster as recommended, RSV vaccine if offered (28-32 weeks).
  • Manage stress — chronic high cortisol affects fetal brain development.
  • Adequate sleep — left side from second trimester onwards.

What this tracker does NOT do

  • Doesn’t assess your individual baby’s growth — only your provider’s scans can.
  • Doesn’t diagnose growth restriction, macrosomia, or any condition.
  • Doesn’t replace prenatal appointments or the anatomy scan.
  • Median figures aren’t targets — being above or below is usually normal.

Sources

  • ACOG patient education materials on fetal development.
  • NHS week-by-week pregnancy guide.
  • Papageorghiou AT, et al. International standards for fetal growth: INTERGROWTH-21st Project. Lancet 2014.
  • WHO fetal growth charts and standards.
  • Hadlock FP, et al. Estimation of fetal weight with the use of head, body, and femur measurements. Am J Obstet Gynecol 1985.
  • BAPM. Perinatal management of extreme preterm birth before 27 weeks of gestation: a framework for practice. 2019.

See our methodology. Educational tracker — not a substitute for medical advice. Read the medical disclaimer.

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Frequently asked questions

How big is my baby right now?
Use the slider above to find your current pregnancy week. Each week shows: a fruit / vegetable size comparison, length in cm/in, weight in g/oz, and a list of what's developing. The figures are 50th-percentile (median) averages — your baby may be slightly larger or smaller and still completely healthy. By week 12 your baby is around plum-sized (5.4 cm, 14 g); by week 20 (anatomy scan week) banana-sized (16 cm, 300 g); by week 40 watermelon-sized (50 cm, 3.4 kg).
When does the baby look like an actual baby on ultrasound?
Recognisable human form starts around week 9-10: head visible, limb buds clear, organs forming. By week 12 most major structures are formed and you can see distinct fingers, toes, and a heartbeat clearly on dating ultrasound. Around week 16-18, facial features become visible. The 20-week anomaly scan shows beautifully detailed views of organs, spine, limbs, and often baby's profile. From week 28+, 4D ultrasound shows recognisable facial expressions.
When can my baby hear me?
Baby's ears begin developing from week 8 but functional hearing starts around week 16-18. By week 25, babies respond reliably to sound — heart rate changes, movements increase. By week 28, babies recognise repeated sounds (familiar voices, music). Talking, reading, singing to your bump from the second trimester onwards has gentle benefits — postnatal studies show babies prefer voices and stories they heard in utero.
When will I feel my baby move?
First movements ('quickening') typically felt 16-22 weeks in first pregnancies, 14-18 weeks in subsequent. Anterior placenta or higher BMI may delay first felt movement. Movements are subtle at first (described as bubbles, gas, flutters) and become unmistakable kicks by 24-26 weeks. From week 28, daily kick counting becomes a useful routine — see /calculators/kick-counter.
Why does my baby's length jump between week 19 and 20?
Measurement convention change, not a sudden growth spurt. UP TO WEEK 19: length is crown-rump length (CRL) — top of head to bottom of buttocks, with legs curled. FROM WEEK 20: legs are extended enough to measure, so length switches to crown-heel length (head to toe). The baby didn't suddenly get much longer — we just started measuring the legs. The numbers jump by 8-10 cm between week 19 and 20 because of this.
How accurate are weekly fetal size charts?
The 50th-percentile median figures are good population averages but individual babies vary widely. A baby measuring at the 30th or 70th percentile at any given week is completely normal. Clinicians watch the TREND of growth across scans (whether baby is tracking consistently along their own curve), not whether any single measurement matches a median. Ultrasound biometry itself has a margin of error of ±10% in the third trimester.
What is the 20-week anomaly scan?
Detailed ultrasound (also called anatomy scan or detailed scan) usually performed between 18-22 weeks. Checks: brain structures, face / lip palate, heart (four chambers, outflows), spine, kidneys, bladder, stomach, abdominal wall, arms / legs / hands / feet, placental position, amniotic fluid volume. Detects most major structural abnormalities. Doesn't detect chromosomal conditions (those need NIPT or amniocentesis). Sex of baby usually identifiable but not the primary purpose.
When is the baby considered viable outside the womb?
23-24 weeks is the conventional viability threshold — the earliest survival is possible with intensive neonatal care. Survival statistics improve substantially with each additional week: 23 weeks ~20-35% survival, 24 weeks ~50-60%, 25 weeks ~70-80%, 26 weeks ~85-90%, 28 weeks ~95%+. UK and most developed-country NICUs offer active resuscitation from 22 weeks onwards in many centres, with parental shared decision-making at 22-24 weeks.
How much weight does my baby gain in third trimester?
A LOT. From week 28 to 40, baby roughly triples in weight from ~1 kg to ~3-3.5 kg. The third trimester is the major fat-deposition phase — brown fat (for temperature regulation), subcutaneous fat (for full-term chubbiness), continued brain growth (~100,000 neurons per hour from 25 weeks), lung surfactant production, and storage of iron / nutrients for the first months of life. This is why preterm babies (especially under 35 weeks) are skinnier and need more nutritional support.
When does the baby's brain develop?
Continuously throughout pregnancy, with major milestones: WEEK 4-6: neural tube forms (folic acid critical). WEEK 8-12: basic brain structure (forebrain, midbrain, hindbrain). WEEK 16-20: most neurons formed; cortex layers emerging. WEEK 20-28: rapid synapse formation; sleep-wake cycles emerge. WEEK 28-40: massive expansion of grey matter, myelination begins, sulci/gyri (brain folds) form. Brain development continues for years after birth — the first 3 years are particularly important.
What does the baby do in there all day?
Lots. From around 16 weeks: practising breathing movements (inhaling amniotic fluid), swallowing (40-150 ml/day by term), urinating (becomes a major component of amniotic fluid), sucking thumb, hiccups (felt as rhythmic taps from week 24+), responding to sounds, light, and your movement. Sleep cycles emerge around 28 weeks — babies sleep ~80% of the time in late pregnancy, often opposite your awake hours (kicks you all night).
Why am I measuring small / large for dates?
Fundal height measurement (top of pubic bone to top of uterus) — in centimetres roughly equals gestational age in weeks from 20 weeks onwards. ±2-3 cm is within normal range. Possible reasons for measuring small: SGA baby, oligohydramnios, transverse lie, breech, retained urine, maternal weight. Possible reasons for measuring large: macrosomia, polyhydramnios, twins (if not known), maternal weight, fibroids, full bladder. NICE recommends growth scan if 3+ cm out from expected.
Is the baby comfortable in there?
Largely yes. The amniotic fluid cushions, the temperature is constant (37 °C), nutrition and oxygen are continuous via placenta. They have moments of discomfort (your sudden movements, your meals, sound vibrations). From around 32+ weeks, space becomes tight as baby grows but the uterus continues to stretch. Many babies position themselves in the same comfortable position repeatedly. Movements help them stretch and exercise developing muscles.
What happens to baby in labour?
Baby experiences the contractions too — but in ways that aren't usually harmful. Each contraction briefly reduces blood flow through the placenta, then resumes; this is the rhythm baby is built for. Baby's head moulds (skull plates overlap slightly) to fit through pelvis. Catecholamines (stress hormones) prepare baby's lungs for first breath and clear lung fluid. Most babies tolerate normal labour well, which is why we monitor with intermittent (low-risk) or continuous (higher-risk) heart-rate monitoring.
How does this relate to other calculators on BumpBites?
Companion: /calculators/due-date for your current pregnancy week; /calculators/pregnancy-week for the milestone view; /calculators/kick-counter for fetal movement tracking from 28 weeks; /calculators/pregnancy-weight-gain for your own weight curve; /calculators/baby-percentile after birth for growth tracking.

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