Baby Size Week by Week: The Complete Guide

One of the most delightful traditions of pregnancy tracking is the weekly fruit comparison. It gives you a tangible sense of how your baby is growing when you can't see them. Here's every week, from tiny seed to full-sized watermelon, with a few fascinating facts along the way.

First Trimester (Weeks 4-13)

Week 4: Poppy seed (1-2mm). A cluster of cells has implanted in your uterine wall. It doesn't look like much yet, but the foundations of every organ system are being laid. Right now, your baby is smaller than a grain of rice.

Week 5: Sesame seed (2mm). The neural tube (which becomes the brain and spinal cord) is forming. A tiny heart tube has started taking shape. Your baby is the size of the seed on top of a burger bun.

Week 6: Sweet pea (4-5mm). The heart begins beating - around 100-120 beats per minute. Arm and leg buds are appearing. A sweet pea is a perfect comparison: tiny, curled, and full of potential.

Week 7: Blueberry (8-11mm). Brain development is accelerating rapidly. The face is starting to form, with dark spots where eyes will be. Three-quarters of babies already show a hand preference this week.

Week 8: Raspberry (14-20mm). Fingers and toes are forming (still webbed). The heart has divided into four chambers and is beating at 150-170 BPM - nearly twice your resting rate. Every major organ system is now present.

Week 9: Cherry (23mm). Your baby is officially called a foetus (previously an embryo). They're already hiccupping - diaphragm contractions that help the lungs develop.

Week 10: Prune (31mm). Bones are beginning to harden. Fingernails are forming. The most critical phase of organ development is nearly complete.

Week 11: Lime (41mm). Tooth buds are appearing under the gums. Your baby can open and close their fists. They're starting to look recognisably human.

Week 12: Plum (54mm, 14g). All major organs are formed and functioning. The first trimester milestone. Many families share their news this week.

Week 13: Peach (65mm, 23g). Vocal cords are developing. Fingerprints are forming - unique patterns that will identify this person for life.

Second Trimester (Weeks 14-28)

Week 14: Lemon (85mm, 43g). Your baby can make facial expressions - frowning, squinting, grimacing. Researchers used 4D ultrasound and discovered babies pull a face when mothers eat kale, but make "laughter-face" reactions to carrots.

Week 16: Avocado (116mm, 100g). The skeleton is shifting from soft cartilage to bone. Hearing is developing. Your baby weighs about as much as a medium avocado - which, coincidentally, is an excellent pregnancy snack.

Week 18: Sweet potato (140mm, 190g). The nervous system is maturing rapidly. Many mothers feel the first flutters of movement this week - tiny kicks and rolls that feel like bubbles or butterflies.

Week 20: Banana (160mm, 300g). Halfway. Your baby has working taste buds and swallows amniotic fluid regularly. If you're carrying a girl, she already has 6-7 million eggs - her lifetime supply.

Week 22: Papaya (190mm, 430g). Skin is becoming less transparent. Touch receptors are active - if you press gently on your belly, baby might push back. The brain is forming at a rate of 100 billion neurons.

Week 24: Corn on the cob (210mm, 600g). Your baby could hear your voice clearly now and can distinguish it from other sounds. They respond to music, loud noises, and the rhythm of your heartbeat.

Week 26: Courgette (230mm, 760g). Eyes are opening for the first time. They can see light filtering through the uterine wall. The lungs are producing surfactant, the substance that will help them breathe air.

Week 28: Large aubergine (250mm, 1kg). REM sleep has started - your baby is dreaming. Most of their day is spent in active sleep. The brain will triple in size between now and birth.

Third Trimester (Weeks 30-40)

Week 30: Cabbage (270mm, 1.3kg). Fat layers are building rapidly, filling out that newborn chubbiness. The bone marrow has taken over red blood cell production. When you laugh, your baby bounces on ultrasound.

Week 32: Jicama (290mm, 1.7kg). Toenails are fully formed. Your baby practises breathing movements 40% of the time, inhaling and exhaling amniotic fluid to strengthen the lungs.

Week 34: Cantaloupe melon (300mm, 2.1kg). The immune system is receiving antibodies from you through the placenta. The nervous system is mature enough to regulate body temperature.

Week 36: Honeydew melon (320mm, 2.6kg). Most babies are head-down by now, getting into position. The skull bones remain soft and unfused - designed to mould during birth.

Week 37: Swiss chard (340mm, 2.9kg). Early term. The firm grasp reflex is ready. Surfactant production is in full swing. Baby could arrive any time from here.

Week 38: Leek (350mm, 3.0kg). Full term. All organ systems are mature. The brain has been adding 20,000-30,000 new neurons every hour throughout the third trimester.

Week 39: Mini watermelon (360mm, 3.2kg). The umbilical cord is about 50cm long and has been delivering 300 litres of blood per day. Your baby's intestines are filled with meconium (their first bowel movement).

Week 40: Watermelon (36-38cm, 3.4kg). Ready. Average birth weight in the UK is 3.4kg for boys and 3.3kg for girls. Their heart has beaten approximately 54 million times. They cry in your accent - they've been listening to the melody of your voice for months.

A note on sizes

Every baby is different. The measurements above are averages based on clinical data, and your baby might be bigger or smaller at any given week. Growth scans track your baby's individual trajectory, and what matters most is consistent growth along their own curve, not matching a textbook number. Average sizes vary across different racial profiles as well.

The fruit comparisons are meant to be fun, not precise. A 20-week baby isn't literally banana-shaped. But holding a banana in your hand and thinking "that's roughly how big my baby is right now" gives you something tangible to connect with during a journey that mostly happens invisibly.