Despite growing calls for gender diversity, the number of women CEOs in the S&P 500 fell by nearly 20% in 2018, according to a report published by The Conference Board, with support from executive search provider Heidrick & Struggles (NASD: HSII).

The study examines CEO succession events announced in the S&P 500 in 2018, comparing historical data that The Conference Board has been collecting since 2001.

At the end of 2018, women held 22 CEO positions in the S&P 500, down from a record 27 in 2017, according to the report. Last year, just one additional woman joined the CEO ranks: Kathy Warden of Northrop Grumman.

“While the number of women CEOs has increased significantly since the report began —  reaching their highest levels in 2017 — the trend has suddenly reversed with the number of women CEOs dropping to pre-2016 levels,” said Bonnie Gwin, vice chairman and co-managing partner of the CEO and board practice at Heidrick & Struggles. “As companies increasingly look inward for their next CEO, companies must do more earlier to develop and train high-potential women leaders and add them to their CEO-ready lists.”

The report also noted nonvoluntary departures of S&P 500 CEOs climbed to 30.5% in 2018, up almost eight percentage points from 2017 — #MeToo-related oustings accounted for five of the 12 dismissals — compared to just one CEO departure for personal misconduct in the S&P 500 from 2013 to 2017.

“With the rise in CEO departures and as the CEO role continues to evolve, we’ve continued to see the need for boards to proactively plan for a range of scenarios and to make succession planning a high priority,” said Jeff Sanders, vice chairman and co-managing partner of the CEO and board practice at Heidrick & Struggles.

The data also found insiders assumed nearly 90% of the open CEO positions in 2018: 52 out of 59 openings. In 2018, departing CEOs had stayed in their roles for an average of 10 years.

After hitting a low of nearly seven years following the Great Recession, CEO tenure has increased almost every year over the past decade. It reached the 10-year mark in three of the past four years.