(Bloomberg) -- WeWork Cos.’s proposal to go public later this year with an all-male board is all too common among new companies -- and it’s diluting gains for women in public company boardrooms.

About 40% of open director roles at S&P 500 companies were filled by women last year. But in public offerings since April, women have only occupied about 18% of the new spots, according to G. Fleck / Board Services, a New York executive recruiter that tracks board changes.

WeWork co-founder and CEO Adam Neumann heads the seven-member board. His co-founder and wife, Rebekah Neumann, and co-founder Miguel McKelvey are senior executives at the company but not on the proposed board. WeWork declined to comment.

In a study of 100 IPO boards from 2014 to 2017, half went public without a single female director, said Mali Gero, co-founder and senior adviser of 2020 Women on Boards, which pressures companies to add more women directors. Data for 2018 isn’t yet complete.

“This really brings down the numbers,” Gero said in an interview. “What we also found is that over time they begin to diversity their boards, but it takes four years. It takes four years for these younger companies to really understand the value of diverse boards and hear what the stakeholders are asking for.”

By that time, though, it’s hard to undo the imbalance. Board seats don’t open up very often. Half of the companies in the Russell 3000 and 43% of those in the S&P 500 had no change to their boards last year, according to a report by the Conference Board Inc.

The IPO boards tend to mirror the venture capital firms that fund them, said George Fleck, of the New York recruiting firm. VC firms are mostly male and have a lot of influence over the composition of the board. Three of WeWork’s six directors were appointed by investors Benchmark Capital, Hony Capital and Softbank Vision Fund LP, WeWork said in its IPO filing.

The other three directors are Lew Frankfort, the former CEO of Coach, Inc.; Steven Langman, the co-founder of global private equity firm Rhone; and Mark Schwartz, a retired Goldman Sachs Inc. executive.

--With assistance from Ellen Huet.

To contact the reporter on this story: Jeff Green in Southfield, Michigan at jgreen16@bloomberg.net

To contact the editors responsible for this story: Janet Paskin at jpaskin@bloomberg.net, Michael Hytha

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