I was one child of two, my father was one of six, my grandmother one of ten, her father one of twelve and her grandfather one of seventeen. Each generation has had fewer children, and that is generally true throughout the country.
Contraception has become possible, acceptable and reliable, infant and child mortality has improved, better nutrition and health care has meant fewer early deaths, safety at work fewer accidents and work-related illnesses, advances in medical knowledge and practice mean that people live longer and more healthily. EDucation and work opportunities for women means they have an alternative to mindless housework and annual childbearing.
It has become gradually less necessary to have a large family to ensure that you have descendants to look after you when you are ill or feeble, and to carry on your name.
People already living here have mostly adapted to the smaller norm, and incomers do sooner or later. This is not an instant process for everyone, but it is the average family size that dictates the overall birth rate.