Baby size by week calculator

How big is my baby this week?

It is the question every pregnancy app answers with a piece of fruit, and there is nothing wrong with the fruit. What is usually wrong is everything around it: an unsourced weight in the first trimester, a length that jumps 10 centimetres between two weeks with no explanation, and no indication of which measurement is being quoted.

This page uses the NHS week-by-week guide for lengths and comparisons, the WHO fetal growth charts for weights from 14 weeks, and names both. Where a number is interpolated it says so.

The two measurements, and the week they swap

For the first half of pregnancy babies are measured crown to rump — head to bottom. The legs are curled up against the body and simply cannot be measured reliably. From 20 weeks the NHS switches to crown to heel, head to toe.

This produces the single most confusing thing in every pregnancy tracker. At 19 weeks the NHS says about 15.3 cm. At 20 weeks it says about 25.6 cm. Nothing happened. The NHS is admirably direct about it on the week-20 page: “Measurements are now taken from head to heel. In earlier weeks, babies are measured from the head to the bottom because the legs are curled up and hard to see.”

If you have ever looked at a size chart and wondered how a baby grew two-thirds again in a week, that is why. Every figure here is labelled with which measurement it is.

Why there is no weight before 14 weeks

Because nobody can give you one honestly, and the people best placed to try have declined.

The NHS publishes no fetal weight at any week of pregnancy. INTERGROWTH-21st, the big international standards project, has a first-trimester standard that measures length only, and a separate weight standard that does not begin until 22 weeks. The WHO fetal growth charts begin at 14.

The reason is mechanical. Fetal weight is never weighed — it is estimated from measurements taken on a scan: the head, the abdomen, the femur. ISUOG’s guideline sets out the practice: “Before 14 weeks, CRL should be used to assess fetal size and to estimate gestational age. After 14 weeks, usual measurements include BPD, HC, AC and FL.” Every weight formula in clinical use needs an abdominal circumference, and at nine weeks there is no abdomen to put a circle around.

One old and much-copied table does run earlier — Hadlock 1991, which gives 35 g at 10 weeks. It is worth knowing where that comes from: 31 subjects covered the entire 10-to-13-week span, and each of their “weights” was itself calculated from the very measurements that cannot be taken at 10 weeks. It is a line extended past the end of its own data. This calculator does not contain those rows, so it cannot show them to you by accident.

Before 14 weeks, then: a length, a fruit, and an honest silence.

The weights disagree, and that is the useful part

From 14 weeks onwards there are several good standards and they do not match. At 32 weeks the WHO 50th centile is 1,901 g. INTERGROWTH-21st says 1,755 g. Hadlock says 1,953 g. Two hundred grams apart, all three built by serious people.

They differ because they were built for different purposes on different populations. WHO pooled ten countries and describes what actually happens. INTERGROWTH-21st is prescriptive: it selected low-risk pregnancies to describe optimal growth. Hadlock is from 392 largely middle-class American patients in the 1980s. None is wrong; they answer slightly different questions.

This page shows all three side by side rather than averaging them into one authoritative-looking number, and uses the WHO figure as its headline.

Reading a centile without worrying

Everything here is a 50th centile. Half of babies at any given week are lighter than it and half are heavier. Being some way off it is normal and expected — and normal in both directions.

Two things are worth knowing. First, a weight from a scan is an estimate with roughly 10 to 15 per cent error built in, so a scan reporting 2,000 g is really saying somewhere between about 1,700 g and 2,300 g. Second, what your midwife or sonographer watches is not one number but the shape of the line over time — a baby who is consistently on the 20th centile is usually just a smaller baby, while one who drops from the 50th to the 10th between scans is the pattern that prompts a closer look.

ISUOG also advises that estimated fetal weight should not be plotted on newborn birth-weight charts. They are a different measurement of a different thing, and mixing them is a common source of unnecessary alarm.

About the fruit

The comparisons here are the NHS’s own — a poppy seed at 4 weeks, a plum at 12, an avocado at 16, a banana at 20, an aubergine at 28, a pumpkin at 40. Where the NHS does not name one for a particular week, this page says so rather than inventing something.

They are also, frankly, approximate. An avocado and a bell pepper are not the same shape as each other, let alone as a baby, and produce varies enormously by country and season. They are here because they help you picture something, not because they are precise. The millimetres are the precise part.

What this cannot tell you

It cannot see your pregnancy. It does not know whether you are carrying twins, who are dated the same way but grow on a different curve. It cannot account for your own height and build, both of which properly influence how big your baby should be. And it is not a growth assessment — that comes from measurements taken on you and your baby, plotted over time, by someone who can look at both.

Use it for orientation and for the small pleasure of knowing this week’s fruit. If you are worried about your baby’s growth, or about movements, ring your midwife or maternity unit. They would always rather hear from you.