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Before birth, a baby develops for about 40 weeks. Scientists divide this into three stages: the germinal stage (weeks 1–2), the embryonic stage (weeks 3–8) and the fetal stage (weeks 9–40).

Babies grow very fast. A newborn's weight typically doubles in six months and triples in one year. Most babies hold up their head at about 6 weeks and sit alone at about 7 months.

Your body proportions change as you grow. In a newborn baby the head is about a quarter of the total length; in an adult it is only about one seventh.

Experiment: Head and body: do proportions change as we grow?

1. Question: is the head a bigger part of the body in babies than in adults? Make a prediction.
2. On the pictures, measure the head height (chin to top of the head) and the total height of each person, in mm.
3. Divide total height by head height. This tells you how many 'heads tall' each person is.
4. Volunteers only: in pairs, measure head height and total height of a volunteer (or your teacher) and work out how many 'heads tall' they are.
5. Fair test: measure everyone the same way (standing straight, no shoes, same chin-to-top method) and measure twice.
6. Compare: a newborn is about 4 heads tall, an adult about 7. Where do the children in your class fit?

Person	Head height (mm)	Total height (mm)	Heads tall (total ÷ head)
Baby (picture)
Child (picture)
Adult (picture)
Volunteer

Questions

1. Describe what your results showed about head and body proportions.

2. Name two things you did to make your measurements fair.

3. Name three changes that can happen in puberty.

4. A baby weighs 3.5 kg at birth. About how much might it weigh at 6 months, when its weight has doubled?

Sources for every fact on this sheet: echoesof.science/school/growing-up/sources/

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