Baby Size Week by Week: The Complete Guide

One of the most common questions in pregnancy is "how big is my baby right now?" It helps to have a reference point - and fruit comparisons have become the standard way to visualise your baby's growth. Here is the complete guide from poppy seed to watermelon.

First trimester: tiny but mighty

At week 4, your baby is about the size of a poppy seed - roughly 1mm. It is a microscopic cluster of cells that has just implanted in your uterine wall. By week 6, they have grown to the size of a lentil (4mm), with a heartbeat and the beginnings of arms and legs.

Growth accelerates rapidly. By week 8 (raspberry, 16mm), fingers and toes are forming. At week 10 (kumquat, 31mm), all vital organs have begun functioning. By the end of the first trimester at week 12 (lime, 54mm), your baby can open and close their fingers and has all major organs formed.

Second trimester: the growth spurt

The second trimester brings dramatic growth in length. At week 14 (lemon, 85mm), your baby can make facial expressions. By week 16 (avocado, 12cm), the skeleton is connecting to the nervous system. At week 18 (sweet potato, 14cm), your baby can hear sounds.

The halfway point at week 20 marks a significant moment: your baby is about the size of a banana (25cm from head to toe) and weighs around 300g. The anomaly scan typically happens this week. By week 24 (corn on the cob, 21cm crown-rump), the baby has reached viability and weighs around 600g.

Third trimester: filling out

The third trimester is all about weight gain. Your baby roughly triples their weight between weeks 28 and 40. At week 28 (large aubergine, 25cm crown-rump), they weigh about 1kg. By week 32 (squash, 28cm), they are putting on about 250g per week.

At week 36 (honeydew melon, 32cm), most babies have moved into the head-down position. By week 40 - your due date - your baby is the size of a watermelon (36-38cm crown-rump), weighing an average of 3.4kg (7.5lbs) in the UK.

Why sizes vary

Every baby grows at their own pace. The sizes listed here are averages, and healthy babies come in a wide range of sizes. Your midwife and sonographer will track your baby's individual growth pattern at scans and appointments. What matters is that your baby is following their own consistent growth curve, not hitting an exact number each week.

Track your baby's growth

Nuhah's free baby size comparison tool and week-by-week pregnancy tracker show you exactly how big your baby is at every stage. From poppy seed to watermelon, we are with you every step of the way.