Baby Size by Week

How this works

Enter the week of pregnancy you are in and the tool shows a familiar size comparison along with typical length and weight for that stage, plus what is developing. The figures are population averages drawn from standard growth references — they describe a typical baby at that week, not your baby specifically.

Before around 20 weeks, length is measured crown to rump (head to bottom), because babies are curled up and legs are tucked. From roughly 20 weeks onward, measurements switch to crown to heel (full length). This is why size charts appear to jump around the halfway mark — the measurement method changed, not the growth rate.

Why size varies so much between babies

Healthy babies at the same gestational age can differ considerably in size, and that variation is normal. Genetics play a large part — taller parents tend to have longer babies. So do your own build, whether this is a first pregnancy, and simple biological variety. A baby measuring somewhat above or below an average chart is usually just following their own growth curve.

This is also why your provider is more interested in the trend across appointments than any single measurement. Consistent growth along a curve is reassuring, even if that curve sits above or below the average line. A single number in isolation says very little.

How growth is actually measured in appointments

From around 24 weeks, your midwife may measure your fundal height — the distance from your pubic bone to the top of your uterus — which roughly corresponds in centimetres to the number of weeks. It is a quick screening measure, not a precise one, and is affected by your build, the baby's position, and the amount of amniotic fluid.

Ultrasound gives a more detailed estimate by measuring the head, abdomen, and thigh bone and combining them into an estimated fetal weight. Even this carries a meaningful margin of error, particularly later in pregnancy, which is why estimates are given as ranges rather than exact figures.

What actually matters between scans

Size charts are interesting, but they are not the thing to monitor at home. Your baby's pattern of movement is the meaningful day-to-day signal of wellbeing. Get to know what is usual for your baby, and if movements reduce or the pattern changes, contact your midwife or maternity unit immediately — at any hour, without waiting for the next appointment and without using a home doppler for reassurance.

For more, see our guides on counting your baby's kicks, healthy pregnancy weight gain, and how far along you are.

Frequently Asked Questions

Is my baby measuring big or small a problem?

Usually not. Healthy babies vary considerably at the same week, largely due to genetics and your own build. Providers watch the trend across appointments rather than a single measurement, and will tell you if anything needs following up.

Why do size charts jump around 20 weeks?

Because the measurement changes. Before about 20 weeks babies are measured crown to rump, since the legs are curled; after that they are measured crown to heel, the full length.

How accurate are ultrasound weight estimates?

They carry a real margin of error, especially in the third trimester, which is why estimates are given as a range. They are useful for spotting trends, not for predicting an exact birth weight.

Should I worry if I measure differently from the chart?

No single number means much on its own. Fundal height in particular is a rough screening measure affected by your build, the baby's position, and fluid levels. Raise any concern with your midwife, who has your full picture.