Growth of any multicellular organism involves increase in size and mass.

Plants and animals grow and develop by various processes:

Cell differentiation is the process in which a cell becomes specialised for a particular function - this increases the efficiency and viability of multi-cellular organisms to survive. (See cell specialisation notes)

Cell division by mitosis (previously described in part 2)

The two points above apply to any multicellular organisms, but plants can also grow by cell elongation - plant cells can expand to grow bigger so the whole plant increases in size.

 

Animal growth and development

All growth in animals occurs by cell division, and most growth happens when the animal is young.

After full growth to be an adult, the animal stops growing.

Therefore when an animal is young you get the fastest rates of cell division - fastest growth rates.

In adulthood most cell division is for 'body repairs', that is growing new cells to replace damaged or dead cells.

In most animals cell differentiation is lost at a relatively early age of development.

 

Percentile charts and monitoring growth of young children

Growth charts (development graphs) are used to monitor a child's development in size or weight to see if there appears to be any problem.

 

Example 1. Babies and young children

Its normal to monitor a baby's growth after birth to see if it is growing normally - three common measurements are used - length, mass ('weight') and head circumference.

You should immediately bear in mind the wide variety of 'sizes' in young children - we are talking 'statistics'.

Doctors will be called in to investigate if the baby's/child's size or weight is above the top percentile or below the bottom percentile - in other words - is there abnormal growth in some way - too much or too little?

Also, if a baby's growth increases or decreases by two or more percentile lines, or the growth pattern is inconsistent, then a medical investigation may be needed.

Using thousands of data sets from many babies you can plot growth charts.

Growth charts are plotted as a series of 'percentile' graph line e.g.

a 50th percentile line will show that 50% of babies/children will have reached a particular value or less of height or weight etc.

Statistically, the 50th percentile line is the median (middle value) of the data set.

The data for boys and girls weight aged 1 to 4 years are shown in the percentile charts above.

Example of percentile chart interpretations:

At age 3, 50% of boys have reached a weight of up to 14.5 kg, so 50% will have a weight of over 14.5 kg..

At age 2, 50% of girls have reached a weight of up to 11.5 kg,  so 50% will have a weight of over 11.5 kg..

 

Example 2. Young children and teenagers

The percentile data on height in cm for boys aged 9 to 18 years (note the 'acceleration' due to puberty, and then a levelling off of the graph into adulthood).

Example of percentile chart interpretation: 75th percentile line

At age 14, 75 % of boys have grown to a height of 168 cm or less, so 25 % of boys are taller than 168 cm aged 14 years.

 

The percentile data on height in cm for girls aged 8 to 18 years (note the 'acceleration' due to puberty, and then a levelling off of the graph into adulthood).

Example of percentile chart interpretation: 25th percentile line

At age 14, 25 % of girls have grown to a height of 157 cm or less, so 25 % of girls are taller than 157 cm aged 14 years.

 

The percentile data on weight in kg for boys aged 9 to 18 years (note the 'acceleration' due to puberty, and then a levelling off of the graph into adulthood).

Example of percentile chart interpretation:

At age 13, 50 % of boys have grown to a weight of 43 kg or less, so 50 % of boys have a weight greater of 43 kg aged 13 years.

 

The percentile data on weight in kg for girls aged 8 to 18 years (note the 'acceleration' due to puberty, and then a levelling off of the graph into adulthood).

Example of percentile chart interpretation:

At age 16, 75 % of girls have grown to a weight of 62 kg or less, so 25 % of girls have a weight greater of 62 kg aged 16 years.

 

Example 3. Comparing percentile trends with each other

You can also do comparison charts of one percentile versus another.

 

 

This is another diagnostic approach to looking for overweight (obese) boys and girls aged 2 to 18

For lots of examples of growth charts from the UK see

and https://www.rcpch.ac.uk/resources/uk-who-growth-charts-0-4-years

https://www.rcpch.ac.uk/sites/default/files/Boys_2-18_years_growth_chart.pdf

 https://www.rcpch.ac.uk/sites/default/files/Girls_2-18_years_growth_chart.pdf

 for downloads to study and from which I prepared the chart diagrams in this section.