The middle of the 20th century was marked by a significant and persistent increase in fertility rates in many countries of the world, especially in the West. The term baby boom is often used to refer to this particular boom, generally considered to have started immediately after World War II, although some demographers place it earlier or during the war. This terminology led to those born during this baby boom being nicknamed the baby boomer generation. The baby boom ended with a significant decline in fertility rates in the 1960s and 1970s, later called the baby bust by demographers.