How big is your baby this week?
This baby size by week chart shows how big your baby typically is in every week of pregnancy, from week 4 to week 40, with an everyday comparison — a lime at 12 weeks, a pomegranate at 20, a pineapple at 28. Choose a week with the slider, or enter your due date to jump to this week, and switch between centimetres and grams or inches and pounds.
From 14 weeks the figures come from the World Health Organization fetal growth charts, built from repeated ultrasound scans of 1,387 low-risk pregnancies in ten countries, including India. They give the average estimated weight and the usual range (10th to 90th percentile), plus the average head circumference and thigh-bone (femur) length. For weeks 6–13 the chart shows the crown–rump length measured on early scans. It is information only, not medical advice: your own baby’s size is judged by your midwife or doctor.
How to use the Baby Size by Week
- Move the week slider or use the arrows; or enter your due date and press Show my week.
- Read the comparison, the average weight and the usual range for that week.
- Choose cm / g or in / lb — your choice is remembered on this device.
- Scroll the week-by-week table and tap a row to jump to that week.
Baby Size by Week features
Weeks 4–40
Every week of pregnancy, with what can and cannot be measured at each stage.
WHO data
Estimated fetal weight percentiles, head circumference and femur length from the WHO multinational study.
Metric or imperial
Grams and kilograms or ounces and pounds; millimetres and centimetres or inches.
Fruit and everyday comparisons
Written to match the average weight from 14 weeks, or the length before that.
From your due date
Works out how many weeks and days pregnant you are today.
How baby size is measured on scans
Until about 13 weeks, a scan measures the crown–rump length (CRL): from the top of the head to the bottom. It is the most accurate way to date a pregnancy, which is why early scans can change your due date. Later, the baby is curled up and too long to measure end to end, so scans measure the head circumference, tummy (abdominal) circumference and femur, and a formula turns them into an estimated fetal weight. That is why this chart gives head and thigh-bone measurements rather than a total length from 14 weeks; length figures you see elsewhere for later weeks are rough estimates.
What the range means
- The average (50th percentile) is the middle value: half of babies are bigger and half smaller at that week.
- The 10th–90th percentile range covers eight in ten babies. A baby outside it is not necessarily unwell — parents’ size, ethnicity, and how accurate the dates are all play a part — but it is a reason for your team to look more closely.
- Estimated weights from scans are estimates with a real margin of error, especially late in pregnancy.
- Countries and hospitals use different charts (WHO, INTERGROWTH-21st, Hadlock and others), so numbers on your report may differ a little from these.
Related tools
Have the measurements from your own scan? The fetal weight calculator turns them into an estimated weight and places it on the same WHO chart. Work out your weeks and due date with the pregnancy due date calculator, follow your weight with the pregnancy weight gain calculator, and from around 24–28 weeks get to know your baby’s movements with the kick counter. Getting ready? The hospital bag checklist and newborn essentials checklist help with the shopping and packing.
Baby Size by Week FAQ
How big is my baby at 20 weeks?
On the WHO charts, the average estimated weight at 20 weeks is 330 g (about 11.6 oz), with most babies between 286 and 380 g — about the weight of a pomegranate. The head circumference averages 17.3 cm.
What size is the baby in cm by week?
Early on, the crown–rump length grows from about 5 mm at 6 weeks to about 5.4 cm at 12 weeks. After that, scans measure the head and thigh bone instead; the table shows both in centimetres for every week.
How much does a baby weigh at 40 weeks in the womb?
The WHO average at 40 weeks is about 3.6 kg (8 lb), and eight in ten babies are between about 3.1 and 4.1 kg.
Are the fruit comparisons exact?
No. Fruit and vegetables vary a lot in size, so the comparisons are a friendly guide matched to the average weight or length, not a measurement.
My scan says my baby is small or big for dates. Should I worry?
Not necessarily. Scan estimates have a margin of error and every chart is a little different. Your midwife or doctor will decide whether you need more checks — ask them what your numbers mean.