Let’s have a think here.
[Scenario One]
Family A lives on a single reasonable wage (say £40k). Father has a bad back but manages to hold down a reasonable job, even though his back gives him ”gip” sometimes. Mother used to work but gave up when she had her first child. They live in a house which they are buying on a mortgage. You could say they are “Middle Class”. They have two children, and would like three more. If they do so, they will have to move to a larger house, and probably need a second or larger car. They will have three more mouths to feed. They will have three more school uniforms to buy, etc. etc. Apart from an increase in Child Benefit (soon to be withdrawn from families with a single income in excess of £40k) their income will remain static. They have to pay for school meals, as well as prescriptions and eye tests for the adults, They pay income tax, National Insurance (soon to go up) and full Council Tax. Lots of other things spring to mind but there’s no space. In short they decide they cannot afford to expand their family.