How big is your baby this week?

How to read this chart

Pregnancy is counted from the first day of your last period, so in weeks 1 and 2 there is no baby yet — conception usually happens around the end of week 2. In weeks 3 to 5 the embryo is smaller than a grain of rice. From about week 8 your baby has a recognisable shape, and from the second trimester onward most of the change is growth: the length grows from under 10 cm to around 50 cm, and the weight rises from tens of grams to around three and a half kilograms.

Fruit comparisons are only a picture to help you imagine the size. Different lists pick different fruit, and the same fruit varies a lot in real life, so do not worry if another app says “lemon” where we say “passion fruit”.

How your baby is measured on a scan

In the first trimester, an ultrasound measures the crown–rump length, which is also the most accurate way to confirm how many weeks pregnant you are. Later scans estimate size from several measurements, such as head circumference, abdominal circumference and thigh bone (femur) length, and combine them into an estimated weight. Those estimates can be off by a noticeable margin, especially late in pregnancy, which is why your care team looks at the trend across visits rather than one number.

If you are told your baby is measuring large or small, ask what that means for your care. Often the answer is simply more monitoring.

Size milestones by trimester

• First trimester (weeks 1–13): from a cluster of cells to about 7 cm; all major organs start forming.
• Second trimester (weeks 14–27): from about 9 cm to over 35 cm head to heel; movements usually become noticeable.
• Third trimester (weeks 28–40): weight more than triples as fat builds up and the lungs and brain mature.

For details on each stage, open your week in the week-by-week guide, or check which month your week falls in.

Common questions