For stocks, the beginning of the year has been marked with volatility -- particularly so for growth stocks. In an extreme example, LinkedIn lost 44% of its value in a single day on Friday when the rapidly growing company reported worse-than-expected guidance of 2016. And two fast-growing companies that are scheduled to report results this Wednesday -- Tesla Motors (NASDAQ:TSLA) and Twitter (NYSE:TWTR) -- could also trade wildly. Indeed, volatility wouldn't be out of the norm for these companies by any means. Twitter and Tesla are down 48% and 36% during the last three months, respectively.

Here's why the market will likely have a close eye on these stocks when they release their fourth-quarter update this week.

Tesla Motors
The hot topic going into Tesla's report is undoubtedly Model X production. With the company hoping the recently launched SUV can essentially help the electric-car maker double its sales, investors are eagerly awaiting the company to ramp-up production of the vehicle. With about 25,000-plus deposit-backed orders for the new vehicle, production is a critical bottleneck for the company's sales growth during 2016.

Model X Falcon Wing Doors

Model X. Image source: Tesla Motors.

During Tesla's fourth quarter, which wrapped up at the end of December, the company delivered about just 208 units of its Model X, compared to 17,192 units of its Model S sedan. With the first deliveries beginning on Sept. 29, Tesla's 208 deliveries during the quarter highlights the automaker's challenges in ramping production.

Notably, the company did say on Jan. 3 that it had actually produced 507 Model X units during the quarter and had planned to deliver those that weren't delivered yet in early Q1. This suggests the company was beginning to ramp up production toward the end of the quarter. But whether the ramp-up continued is still an unknown.

Expect the company to provide an update on Model X production when it reports fourth-quarter results -- and possibly even specific guidance for how many SUVs it expects to produce during the quarter.

Twitter
For Twitter stock, investors will be eyeing the company's user growth and anything that has to do with the company's projections for user growth, going forward. With user growth decelerating in recent quarters to as low as 1% sequentially for most recent reported quarter, investors are concerned the platform may not have the mass-market appeal the market was hoping for.

Twitter Homepage Desktop

Twitter's home page for desktop. One of Twitter's recent moves aimed at attracting new users was the revamp of its home page for the logged out experience. Image source: Twitter.

Twitter CEO and co-founder Jack Dorsey, who returned to the company as CEO last year, recognizes that the social media company's user growth is a problem and is attempting to address the issue. But with the stock falling more than 40% during the three months leading up to the company's results, investors seem unconvinced the social network can reinvigorate user growth.

Investors can check in on the company's reported users for Q4, as well as any forward-looking comments about user growth and what the company expects from the important metric. Twitter currently has 320 million monthly active users. If the company fails to reinvigorate user growth slightly during the quarter -- perhaps to at least 2% sequential growth -- there's a good chance investors may be worried about Dorsey's ability to turn things around.

Both companies report earnings after market close on Wednesday. For a closer look at what to expect from these companies when they report results, check out these earnings previews for Tesla and Twitter.

Daniel Sparks owns shares of Tesla Motors. The Motley Fool owns shares of and recommends LinkedIn, Tesla Motors, and Twitter. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.