The average life expectancy in the U.S. has been on the decline for three consecutive years.

A baby born in 2017 is expected to live to be 78.6 years old, which is down from 78.7 the year before, according to data from the Centers for Disease Control and Prevention's National Center for Health Statistics.

The last three years represent the longest consecutive decline in the American lifespan at birth since the period between 1915 and 1918, which included World War I and the Spanish Flu pandemic, events that killed many millions worldwide.

Before the recent decline, life expectancy had been steadily rising in the U.S. — which is to be expected of an advanced nation, particularly one that spends more money per citizen on health care than any other country.

The U.S. isn't alone. A study projected the U.K. lifespan will shorten by about five months. While life expectancy is still on the rise in France, Germany, Sweden and the Netherlands, those countries have also seen a sharp slowdown.

While there's no single cause for the decline in the U.S., a report by the CDC highlights three factors contributing to the decline.