The 585 water mains that broke or leaked in the Maryland suburbs during January’s bitter cold achieved a dubious distinction: The monthly tally was the fourth-highest in the past 20 years, utility officials said Wednesday.

In first place for Worst Month Ever: December 2010, with 645 breaks and leaks. The second-worst was January 2003, with 599 pipe problems, and third-worst was January 2009, with 590, according to the Washington Suburban Sanitary Commission, which supplies water to Montgomery and Prince George’s counties.

The aging pipes — some are nearly a century old — weaken and break when the temperature of the water inside them fluctuates. The water inside the pipes comes from the Potomac and Patuxent rivers, where swings in water temperatures usually lag behind air temperatures by a few days. Rising or dropping water temperatures stress the pipe by causing weakened pipe walls to expand or contract.