The first U.S. Bitcoin IPO will be a major sensation on Wall Street when it arrives.

The big question is when that day will come, and which of several contenders will go public first.

A freshly minted Bitcoin stock will give investors another way to invest in Bitcoin, much as gold mining stocks are an alternative way of investing in gold.

Right now the options for Bitcoin are limited. There aren't any Bitcoin stocks and two Bitcoin ETFs, the Winklevoss Bitcoin Trust (BATS: COIN) and the SolidX Bitcoin ETF (NYSE Arca: XBTC) are awaiting regulatory approval. [Read More: Why the Winklevoss Bitcoin ETF Is Ahead in the Race for SEC Approval]

That's a big reason why a Bitcoin IPO would be such a big deal. And several Bitcoin startups have reached the point where a U.S. Bitcoin IPO could arrive in less than two years.

The key is to watch the flow of venture capital, which has poured steadily into cryptocurrency-based businesses for about four years now…

Why the Wait for the First Bitcoin IPO Won't Be Much Longer

Several prominent Bitcoin startups have in fact advanced to the later rounds of venture capital investing, making them prime candidates to become the first U.S. Bitcoin IPO.

You see, venture capital investing runs through roughly five stages. It begins with a "seed round." As the company shows promise and a need for more capital, it gains more funding in successive rounds.

After the fifth stage, a company is either acquired or goes public to obtain capital from the markets. According to a CB Insights study in 2013, the average time for Internet companies to exit after their first round of funding is a little over four years.

Three of the top four prospects to become the first U.S. Bitcoin IPO received their first funding in 2012 and 2013, which suggests at least one of them will exit within the next year to 18 months.

Here's a closer look at the Bitcoin companies most likely to be the first U.S. Bitcoin IPO…

Which of These Four Startups Will Be the First U.S. Bitcoin IPO?

Bitcoin IPO Prospect No. 1: Coinbase

Coinbase has a strong case to become the first Bitcoin IPO. Founded in June 2012, Coinbase received its first round of venture capital funding in September of that year, making it the oldest company of the four listed here. The San Francisco-based company runs a cryptocurrency exchange for both Bitcoin and Ethereum as well as a digital wallet service.

A fourth-round $10.5 million investment in July, led by the Bank of Tokyo, was Coinbase's most recent infusion of venture capital. According to CrunchBase, that brings the company's total VC funding to $117.21 million. Coinbase has an estimated valuation of about $471 million.

Bitcoin IPO Prospect No. 2: 21 Inc.

The once-mysterious Bitcoin company 21 Inc. received its first funding in November 2013. It has created an embeddable mining chip that allows any Internet-connected device to mine tiny amounts of Bitcoin that could be used to make micropayments. This year 21 Inc. released software for Mac and Linux PCs to enable Bitcoin mining.

Based in San Francisco, 21 Inc. has received a total of $121.05 million in venture capital funding. Its most recent round of funding was a $116 million Series C investment led by Andreessen Horowitz. There is no estimated valuation for 21 Inc.

Don't Miss: Inspired by the two SEC filings for Bitcoin-based ETFs, a group in July filed to create an Ethereum-based ETF. But it may have jumped the gun…

Bitcoin IPO Prospect No. 3: Digital Asset Holdings

Frankly, the odds are against Digital Asset Holdings becoming the first Bitcoin IPO, as it only received its initial VC funding in February of this year. But because it is working directly with Wall Street and landed former JP Morgan Chase & Co. (NYSE: JPM) superstar Blythe Masters as its CEO, Digital Asset will be a monster IPO. And if the others lag, it could even become the first Bitcoin IPO.

New York-based Digital Asset is focused on harnessing the power of the blockchain, the technology that underpins Bitcoin, to build solutions that make financial settlements faster and more efficient. In June it received $7.2 million in Series A funding from the Australian Securities Exchange (ASX). Digital Asset's total VC investment stands at $67.2 million. The company is valued at about $100 million.

Bitcoin IPO Prospect No. 4: Circle Internet Financial

Boston-based Circle Internet Financial received its initial round of VC funding in October 2013. Circle is a P2P mobile payments app that uses blockchain technology and other Internet protocols to allow customers to move money almost anywhere quickly and cheaply.

In June, Circle received $60 million in Series D funding in a round led by IDG Capital Partners. That brings Circle's total VC funding to $136 million, and its valuation to $480 million. Both of those figures are the highest among all U.S. Bitcoin startups. Given that level of VC interest, Circle probably has a slight edge over the others in becoming the first U.S. Bitcoin IPO.

Up Next: What This Court Ruling Means for Bitcoin's Future – and Your Money

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