The cost of children by Ruth Mace, explain the "apace" as it is used in the opening paragraph of the passage.

The word "apace" in this paragraph means "quickly" or "swiftly". The author is talking about birth rate declines, not only in the Western world, but all around the world; it is a trend that is not going to disappear or slow down anytime soon, and it is conquering the world at a pace that is becoming faster every moment. In other words, the trend is growing in speed.