About the Baby Size by Week
This baby size by week tool answers the question every expecting parent asks — “how big is my baby right now?” Pick a week of pregnancy, or enter your due date and today’s date, and it shows your baby’s approximate length in centimetres and inches, weight in grams and pounds/ounces, and a familiar fruit or vegetable of a similar size. A full week-by-week chart from 8 to 42 weeks is included.
It is meant for parents following their pregnancy week by week, for partners and grandparents-to-be, and for anyone writing a pregnancy update or announcement. Values between whole weeks are interpolated by day, so 24 weeks 3 days falls between the 24- and 25-week figures.
The measurements are typical averages used in prenatal education, not medical reference ranges. Until about 20 weeks a baby is measured from crown to rump because the legs are curled up; from 20 weeks the figures are crown-to-heel, which is why length jumps at that point. Real babies vary a lot — your ultrasound estimate may be weeks ahead or behind and still be completely normal. Weeks are counted from the first day of your last period, as doctors do.
How to use the baby size by week
- 1Choose whether to enter your week of pregnancy or your due date.
- 2Enter the weeks and days pregnant, or your due date and today’s date.
- 3Read the fruit comparison, length and weight for this week.
- 4Scroll the week-by-week table to see how your baby will grow next.
Formula and method
Gestational age (GA) is counted in days from the first day of the last menstrual period. With a due date, the tool subtracts the days remaining from 280 (40 weeks). The length and weight for the completed week come from a table of typical averages; extra days are filled in by straight-line interpolation towards the next week’s value.
Weeks 8–19 use crown-to-rump length and weeks 20 onwards use crown-to-heel length, so the length is not interpolated across week 19 to 20. Inches are centimetres ÷ 2.54 and ounces are grams ÷ 28.35. The fruit comparison is a friendly approximation of length and volume, not a precise measurement.
- GA
- Gestational age in days from the last menstrual period
- V(week)
- Average length or weight for a completed week
- 280
- Days from last period to due date (40 weeks)
Worked examples
Halfway: 20 weeks
At 20 weeks the average baby is about 25.6 cm (10.1 in) from head to heel and weighs about 300 g, or 300 ÷ 28.35 ≈ 10.6 oz — roughly the size of a banana.
28 weeks 3 days
Three days into week 28 is 3/7 of the way from the 28-week figures (37.6 cm, 1,005 g) to the 29-week figures (38.6 cm, 1,153 g): 37.6 + 0.43 × 1.0 ≈ 38.0 cm and 1,005 + 0.43 × 148 ≈ 1,068 g, about 2 lb 5.7 oz.
From a due date of January 15, 2027
On September 29, 2026 there are 108 days left until January 15, so gestational age is 280 − 108 = 172 days, or 24 weeks 4 days. Interpolating 4/7 of the way from 600 g to 660 g gives about 634 g — about the size of an ear of corn.
Frequently asked questions
How big is my baby at 12 weeks?+
At 12 weeks the average baby is about 5.4 cm (2.1 in) from crown to rump and weighs around 14 g (half an ounce) — roughly the size of a lime.
Why does baby length jump at 20 weeks?+
Before 20 weeks the legs are curled against the body, so babies are measured from the top of the head to the bottom (crown-rump). From about 20 weeks charts switch to head-to-heel length, which adds the legs.
My ultrasound says my baby is bigger or smaller than this — is that a problem?+
Not necessarily. Babies vary, and ultrasound weight estimates can be off by 10–15% or more. Your midwife or doctor tracks growth over several scans and against medical growth charts, which is more meaningful than one number.
How much does a baby weigh at birth?+
Most full-term babies weigh between about 2.5 kg and 4 kg (5.5 to 8.8 lb). The average at 40 weeks in the table here is about 3.5 kg (7.6 lb).
Are pregnancy weeks counted from conception?+
No. Pregnancy weeks are counted from the first day of your last period, about two weeks before conception. A baby at “20 weeks” has been developing for about 18 weeks.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.