Billion-dollar “unicorn” start-ups are rushing to take advantage of the bull market and sell their shares to the public, even if they’re losing piles of money.

And big investors such as Vanguard Group, the Malvern-based $5 trillion-asset fund company, haven’t waited for those unicorns to go public. They acquired preferred shares in firms such as Uber Technologies, WeWork, and AirBnB, sold privately by some of the earliest investors in these hottest names in Silicon Valley.

They buy these stocks and post estimates of what they may be worth alongside the publicly traded stocks they own, boosting the valuations — or cutting them — in response to updates on the firms’ prospects and stock market conditions.

This morning, for example, Vanguard posted valuations backdated to Feb. 28 for ride-share service Uber, which it valued at $43.44 a share, up 24 percent over the last year; and shared workspace provider WeWork, which it priced at $68, up 31 percent from Vanguard’s previous estimate. The price revisions were part of the periodic shareholder report to owners of the Vanguard U.S. Growth Fund.

By comparison, the Standard & Poor’s 500 Index rose a little more than 7 percent in the same period.

Both Uber and WeWork have filed with the U.S. Securities and Exchange Commission to go public this year.

Still, Vanguard’s private-company analysts aren’t always relentless optimists.

In the same Feb. 28 filing, they valued apartment-rental giant Airbnb at just under $110 a share, a 4 percent gain over the last year, trailing the market. And social-media photo-sharing service Pinterest was marked down 26 percent over year-ago estimates, to $5.13 a share.

But on Vanguard’s own website, the company has lately adjusted those valuations, too, noted Daniel P. Wiener, chief executive of Adviser Investments and a longtime Vanguard-watcher.

Since Feb. 28, Vanguard has boosted its valuation of Uber preferred shares another 29 percent, to $56 a share; raised Airbnb preferred 16 percent, to more than $127 a share; and Pinterest preferred by 11 percent, to $5.70, according to data filed on the company’s online page for U.S. Growth fund investors. (Pinterest has since sold common stock in its April 15 IPO; those shares closed Tuesday at $30.98, down $3.28 (9.57 percent).

The Uber and WeWork revisions, coming so close to the companies’ planned IPOs, will “probably be the last valuation estimate Vanguard makes” before the stock market takes over pricing, Wiener added.

Vanguard has an “independent committee,” not its fund managers, which sets the “fair market value” of its private investments, spokesperson Emily Farrell said. It considers “financial performance, corporate actions, offering price, market conditions, and guidance from external advisers” in estimating values.

Uber hopes for a public-market valuation of $100 billion — about half what Comcast, a vastly larger company, is worth, and a lot for a company that admits to losing $150 million a month on its operations last year.

WeWork is, by some measures, even more ambitious.

The company, which recently rebranded itself the We Company, said it would diversify into apartments, child care, and software training, and hopes to be valued at nearly $50 billion. That would take a lot of faith in the future of a company that lost $1.9 billion after sales of just $1.8 billion last year, pointed out Matt Topley, who heads the investment committee at King of Prussia-based Fortis Wealth, in his daily client newsletter.

It’s a neat trick, to be able to juice mutual-fund values with private-company stocks. But the impact, for now, is limited: In the case of U.S. Growth, the private-company preferred stocks amount to only about 1 percent of the publicly traded, common stocks held in the same portfolio.