Complex sentence: The proportion of all illegitimate births occurring to teenagers had more than doubled since the 1950s from 15 to 37%, whilst the proportion occurring to women over the age of 25 had fallen from 56 to 32% in the same period.
Split into simpler sentences: The proportion of all illegitimate births occurring to teenagers had more than doubled since the 1950's from 15 to 37%. Whilst the proportion occurring to women over the age of 25 had fallen from 56 to 32% in the same period.