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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 Western world. 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 boom coincided with a marriage boom. The increase in fertility was driven primarily by a decrease in childlessness and an increase in parity progression to a second child. In most of the Western countries, progression to a third child and beyond declined, which, coupled with aforementioned increase in transition to first and second child, resulted in higher homogeneity in family sizes. The baby boom was most prominent among educated and economically active women. The baby boom ended with a significant decline in fertility rates in the 1960s and 1970s, later called the baby bust by demographers.