A sharp decline in the US stock market during the final quarter of 2018 dashed hopes that the nations largest corporate pension plans would enjoy consecutive years of improved funding.
Willis Towers Watson (WLTW) estimates that over a third of Fortune 1000 firms had an aggregate pension funded status of 84% by the end of 2018, down from 85% one year earlier.
This is despite the funding status reaching 90% after the first nine months, with a miserable final quarter for US stocks offsetting these improvements.
WLTW managing director, Royce Kosoff, said the "seesaw year" for funding over 2018 highlights why sponsors must review their overall pension management strategy in 2019.
"The volatility in the fourth quarter, especially in December - one of the worst months since the Great Recession - demonstrates how quickly conditions change," he added.
According to the WLTW analysis, total pension plan assets for the 389 Fortune 1000 companies studied declined from $1.48trn (£1.16trn) to $1.33trn over 2018.
Overall investment returns are estimated to have averaged a negative 4.7% last year, although this varied significantly by asset class.
Domestic large capitalisation equities lost 4%, while small and mid-capitalisation equities realised losses of 10%.
Aggregate bonds provided no return, while long corporate and long government bonds, typically used in liability-driven investing strategies, experienced losses of 7% and 2%, respectively.
The companies studied are thought to have contributed $47bn to their pension plans in 2018.
WLTW said many sponsors had taken advantage of deductions at the older, higher tax rates that existed before tax law changes were introduced by the Trump administration.
Pension obligations declined from $1.74trn in 2017 to an estimated $1.59trn last year, with the total pension deficit forecast to have fallen from $260bn to $255bn.
"We expect sponsors will continue to express interest in risk management strategies, such as revisiting their investment approach or transferring obligations," Kosoff added.