The funded status of big U.S. defined benefit pension plans has improved in the past year, but the recent drop in interest rates could cause new problems.

Analysts at Milliman Inc., Seattle, have reported those findings in their latest pension funding report.

The analysts create the report by analyzing data from 100 large U.S. pension plans.

The plans’ funded status increased to 87.2% at the end of April, up from 82.7% a year earlier. The plans’ cumulative asset return was about 10% over that period.

The plans included in the analysis ended April with about $1.2 trillion in assets and about $1.4 trillion in pension liabilities.

Pension plan money managers invest heavily in corporate bonds, government bonds and other debt securities. Interest rates fell to 5.37% in April, from 5.53% just a month earlier. If interest rates stay low, that could lead to a significant increase in the plans’ liabilities, the Milliman analysts say.

- Allison Bell

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