Pension plan funding among some of the largest U.S.-based companies regressed in April, marking the first downward turn since the end of 2011, according to a Mercer L.L.C. report released on Wednesday.

Pension plans sponsored by S&P 1500 companies incurred a $73 billion rise in their aggregate deficit over totals from a month ago, to $409 billion from $336 billion in March, the report said. The deficit growth returned the plans' aggregate funded ratio to 79%, after the ratio had risen to 82% last month.

In a statement released on Wednesday, Mercer analysts pinned the widening deficit to an increase in liabilities due to declining interest rates. In particular, interest rates on high-quality corporate bonds fell by as many as 32 basis points in April, while assets remained flat as equity markets in the U.S. dipped slightly by about 0.6%.

“After three straight months of improvements in funded status, April saw a bit of a step back for U.S. pension plans,” said Jonathan Barry, a partner in Mercer's Retirement Risk and Finance business, in the statement. “It's an important reminder to plan sponsors that these plans can go down just as quickly as they went up.”

In an effort to reduce volatility, Mr. Barry said he expects plan sponsors to continue seeking out new risk management strategies, including higher fixed income allocations and cash-outs for former employees.

“For the past few years, many sponsors have been slow to act on some of these strategies, as there was an expectation that interest rates would rise, but perhaps we are seeing sponsors come to grips with a potential prolonged period of low rates,” Mr. Barry said. “While cash-outs to retirees may not be a fit for every plan sponsor, it is another example of a company taking a major step to manage their pension risk.”

Despite the slump in April, Mercer's report noted that the S&P 1500-sponsored plans' funding levels are still substantially higher than they were at the end of 2011. Estimated aggregate assets at the end of December 2011 were approximately $1.45 trillion, compared with an estimated $1.56 trillion on April 30, 2012.