The proportion of prime-age women in the U.S. workforce jumped in August to its highest level in more than 17 years, reversing a surprising decline since the start of the year.

Labor force participation for women 25 to 54 years old advanced to 76.3%, up a full percentage point from July and the highest since February 2002, the Labor Department’s employment report showed earlier this month. Participation had been declining since reaching 76% in January despite the lowest overall unemployment rate in almost half a century.

“In some ways it was more perplexing that it had fallen earlier in the year in a very strong labor market,” said Julia Coronado, president and founder of MacroPolicy Perspectives LLC. “It’s more of a catch-up to where we should have been.”

Coronado said the jump might have been modestly boosted by hiring for the U.S. census but not enough to substantially explain the rise. A Labor Department report earlier this week projected prime-age women labor participation at 76.1% in 2028.

Participation among prime-aged men was little changed, rising 0.1 percentage point to 89% in August.

The Labor Department’s survey of households also showed that the unemployment rate among blacks fell to a record-low of 5.5%, although participation among black men 20 years and older eased 0.1 percentage point to 68.3%.

Participation is the share of people working or actively seeking jobs divided by the civilian population aged 16 and over.

With assistance from Sophie Caronello.