The short answer: a baby grows from about a millimetre and a half at week 3 to roughly 51 cm and 3.6 kg at week 40. Useful landmarks along the way: week 12, about 5.5 cm and 12 g, the size of a lime; week 20, about 26 cm and 300 g, a banana; week 28, about 38 cm and 1.1 kg, an aubergine. Up to week 13 the length is measured crown to rump; from week 14 it is crown to heel, which is why the figure jumps in that week without anything unusual happening.
Two different rulers: crown-rump and crown-heel
The single most confusing thing on any fetal size chart is the step at 14 weeks. It is not a growth spurt. It is a change of measurement:
- Weeks 3–13 — crown-rump length (CRL). Head to bottom, with the legs curled up against the body and not included. This is the measurement a dating scan uses, and it is the most accurate way to date a pregnancy there is.
- Weeks 14–40 — crown-heel length. The full head-to-toe length, the way a newborn is measured on the day of birth.
So a 13-week fetus measuring 7.5 cm and a 14-week fetus measuring 14 cm have not doubled in a week — the second figure simply includes the legs. Every chart on every pregnancy site does this, and most of them do not say so.
The full chart: every week, length and weight
The same forty rows as the pictures above, as plain numbers. Each week links to its own page, where the same figures sit alongside what is developing that week and which tests fall around it.
| Week | Baby’s length | Baby’s weight | About the size of |
|---|---|---|---|
| Week 1 | — | — | — |
| Week 2 | — | — | — |
| Week 3 | 1.5 mm | — | a poppy seed |
| Week 4 | 1.5 mm | — | a poppy seed |
| Week 5 | 2 mm | — | a sesame seed |
| Week 6 | 4 mm | — | a lentil |
| Week 7 | 9 mm | — | a blueberry |
| Week 8 | 1.5 cm | — | a raspberry |
| Week 9 | 2.3 cm | — | a grape |
| Week 10 | 3.3 cm | 4 g | a strawberry |
| Week 11 | 4.5 cm | 7 g | a fig |
| Week 12 | 5.5 cm | 12 g | a lime |
| Week 13 | 7.5 cm | 22 g | a lemon |
| Week 14 | 14 cm | 40 g | a peach |
| Week 15 | 15 cm | 70 g | an apple |
| Week 16 | 18 cm | 110 g | an avocado |
| Week 17 | 20 cm | 150 g | a pear |
| Week 18 | 22 cm | 205 g | a sweet potato |
| Week 19 | 24 cm | 240 g | a mango |
| Week 20 | 26 cm | 300 g | a banana |
| Week 21 | 27 cm | 360 g | a carrot |
| Week 22 | 28 cm | 430 g | a papaya |
| Week 23 | 29 cm | 500 g | a grapefruit |
| Week 24 | 30 cm | 600 g | a corn cob |
| Week 25 | 32 cm | 700 g | a swede |
| Week 26 | 34 cm | 850 g | a courgette |
| Week 27 | 36 cm | 1.0 kg | a cauliflower |
| Week 28 | 38 cm | 1.1 kg | an aubergine |
| Week 29 | 39 cm | 1.2 kg | a butternut squash |
| Week 30 | 40 cm | 1.4 kg | a large cabbage |
| Week 31 | 41 cm | 1.5 kg | a coconut |
| Week 32 | 42 cm | 1.7 kg | a large daikon radish |
| Week 33 | 43 cm | 1.9 kg | a pineapple |
| Week 34 | 44 cm | 2.1 kg | a cantaloupe melon |
| Week 35 | 46 cm | 2.4 kg | a honeydew melon |
| Week 36 | 47 cm | 2.6 kg | a head of romaine lettuce |
| Week 37 | 48 cm | 2.9 kg | a stalk of Swiss chard |
| Week 38 | 49 cm | 3.1 kg | a leek |
| Week 39 | 50 cm | 3.3 kg | a small pumpkin |
| Week 40 | 51 cm | 3.6 kg | a small watermelon |
Weeks 1 and 2 carry no measurement because there is no embryo yet — pregnancy is dated from the last period, so those two weeks precede conception. Weight is not routinely quoted before week 10; a dash means "not measured this early", never zero. The conception date calculator explains the two-week offset.
What growth looks like, trimester by trimester
First trimester — everything is built
From a poppy seed to a lime. Almost nothing of this trimester is about size; it is about structure. The neural tube closes by around week 6, the heart is beating by week 6, and by the end of week 12 every major organ system is formed and the rest of pregnancy is about growth and maturation. This is also when a dating scan gives the most accurate gestational age you will ever get. See the first trimester guide.
Second trimester — length outpaces weight
From a lime to a cauliflower. The baby roughly triples in length across this trimester while the weight climbs from grams into hundreds of grams. The anomaly scan at 18–21 weeks is the detailed structural check, and first movements are usually felt somewhere between 16 and 24 weeks. See the second trimester guide.
Third trimester — weight outpaces length
From a cauliflower to a small watermelon. Length gains slow while weight roughly triples: most of what a baby puts on after week 28 is fat and muscle, which is what makes late scans mostly about growth rather than anatomy. From week 28 daily kick counts begin. See the third trimester guide.
Why your baby will not match this chart exactly
These are population averages and healthy babies scatter widely around them. The things that move a baby off the average are mostly not problems:
- Genetics. Tall parents have longer babies. This is the single largest factor.
- Parity. Second and subsequent babies are on average slightly heavier than firstborns.
- Sex. Boys are on average marginally heavier at birth than girls.
- Dating error. If the due date is out by a week, every measurement will look a week off.
- Multiples. Twins grow along a different curve and are typically smaller from the late second trimester.
- Maternal health. Gestational diabetes tends towards larger babies; high blood pressure, pre-eclampsia and smoking towards smaller ones.
When size is actually a clinical question
Clinicians do not compare a baby against a table. They compare a baby against its own trajectory, plotted on a growth chart across several scans. A baby that has measured on the 10th percentile all along is usually just small; a baby that was on the 50th and has dropped to the 10th is the one that gets attention.
Two things worth knowing about the numbers you will be given:
- Estimated fetal weight is an estimate. The formula combining head, abdomen and femur measurements carries an error of roughly 10–15% either way. A late scan cannot tell you a birth weight, and "the scan says he's big" is not on its own a reason for anything.
- Fundal height is a screening tool, not a measurement of the baby. The tape measure from pubic bone to the top of the uterus is affected by fluid, position and your own build. It is used to decide who needs a scan, not to decide anything by itself.
The fetal growth percentiles guide covers what P5, P50 and P95 mean and when a small measurement is genuinely a concern, and ultrasound results explained decodes every abbreviation on the report.
When to call, regardless of size
Contact your midwife or maternity unit the same day — not at the next appointment — if you have:
- A change in your baby’s movements after 28 weeks, especially a reduction. Never wait until morning, and never try to "wake the baby up" with cold drinks or sugar first.
- Vaginal bleeding at any stage
- A sudden increase in swelling, a severe headache or visual disturbance — the signs of pre-eclampsia
- Fluid leaking from the vagina before 37 weeks
- Regular painful tightenings before 37 weeks
Reduced movement is the one on this list that gets left the longest, and it is the one where waiting matters most.
Want the dates as well as the sizes? The pregnancy calendar takes your due date and fills in the calendar dates of all forty of these weeks, with every screening window marked. If you are not sure of your dates yet, start with the due date calculator.
Related tools and guides
FAQ — Baby size week by week
How big is my baby right now?
Find your week in the chart above. As a few landmarks: at 8 weeks an embryo is about the size of a raspberry, at 12 weeks a lime, at 20 weeks a banana, at 28 weeks an aubergine and at 40 weeks a small watermelon. Every week on this page carries the actual length and weight alongside the comparison.
Why does the length jump so much at 14 weeks?
Because the measurement changes. Up to 13 weeks a fetus is measured crown to rump — head to bottom, with the legs curled up and not counted. From 14 weeks it is measured crown to heel, the full length. Nothing dramatic happens to the baby in that week; the ruler moves. This is the standard obstetric convention and every growth chart uses it.
Are these numbers averages or exact?
Averages. Healthy babies vary enormously, and by the third trimester the spread between a small and a large baby of the same gestational age is measured in whole kilograms. A baby measuring above or below these figures is usually just a baby, not a problem. What clinicians watch is the pattern across scans, not one number against a table.
How is the baby measured before birth?
By ultrasound. Early on it is a single crown-rump measurement. From the second trimester the sonographer measures several dimensions — head circumference, biparietal diameter, abdominal circumference and femur length — and a formula combines them into an estimated fetal weight. That estimate carries an error of roughly 10 to 15% either way, which is why a scan cannot tell you a birth weight.
Does a big bump mean a big baby?
Not reliably. Bump size depends on your build, your abdominal muscles, how much amniotic fluid there is, the baby’s position, whether this is a first pregnancy and where the placenta sits. Midwives measure fundal height in centimetres from the pubic bone precisely because eyeballing a bump is not a measurement.
When is the baby’s size actually a concern?
When growth deviates from its own trajectory rather than from a table. A baby who was tracking on the 50th percentile and drops to the 10th is more significant than one who has measured on the 10th all along. Persistent measurements below the 10th percentile or above the 90th are followed up with extra scans. Our fetal growth percentiles guide explains what P5, P50 and P95 actually mean.
Why do the fruit comparisons differ between websites?
Because nobody standardises fruit. A "mango" is a different object in three different countries, and sites pick comparisons their own readers will recognise. Treat them as a friendly mental picture; the centimetres and grams next to them are the part that carries information.
What size is the baby at each trimester?
By the end of the first trimester (13 weeks) a fetus is around 7.5 cm crown to rump and weighs about 22 g. By the end of the second (27 weeks) it is about 36 cm crown to heel and 1.0 kg. At 40 weeks the average is roughly 51 cm and 3.6 kg.
Sources
Baby size week by week — at a glance
A baby grows from around a millimetre and a half at week 3 to roughly 51 cm and 3.6 kg at week 40, passing about 5.5 cm and 12 g at week 12, 26 cm and 300 g at week 20, and 38 cm and 1.1 kg at week 28. Length is measured crown to rump up to week 13 and crown to heel from week 14, which is the reason the figure appears to jump in that week. Weeks 1 and 2 carry no measurement because pregnancy is dated from the last menstrual period and those weeks precede conception. Every figure here is a population average: genetics, parity, sex, dating accuracy and maternal health all move a healthy baby off the mean, and estimated fetal weight from a scan itself carries an error of roughly 10 to 15 per cent. What matters clinically is a baby's own trajectory across several scans rather than a single measurement against a table — which is what percentiles describe. Regardless of size, a change in movements after 28 weeks is always a same-day call.