Complex sentence: Saudi population has grown rapidly since 1950 when it was estimated to be 3 million, and for many years had one of the highest birthrates in the world at around 3% a year.
Split into simpler sentences: Saudi population has grown rapidly since 1950 when it was estimated to be 3 million,. United Nations and for many years had one of the highest birthrates in the world at around 3% a year.