Welcome to Health Reform Watch, Sarah Kliff’s regular look at how the Affordable Care Act is changing the American health-care system — and being changed by it. You can reach Sarah with questions, comments and suggestions here. Check back every Monday, Wednesday and Friday afternoon for the latest edition or sign up here to receive it straight from your inbox. Read previous columns here.

This past February, well before HealthCare.gov opened for business, the Congressional Budget Office made a prediction. The nonpartisan agency projected that the federal exchange, alongside 14 state run Web sites, would sell 7 million people insurance policies in 2014.

The number quickly became the political benchmark for the health law's success or failure, a ruler by which to measure the success of a largely unprecedented insurance expansion.

White House officials used it when they talked about how many young people they needed to sign up. Health and Human Services forecasted out, month by month, how many sign-ups they would need to get to 7 million.

Health law opponents, too, focused on the CBO projection when they saw terrible sign-up numbers in October and November. How could you get to 7 million sign-ups, after all, when the first day of enrollment had netted a mere six customers?

But what seemed impossible in October suddenly became a lot more plausible in late December. This weekend, new enrollment data showed approximately 2 million Americans signed up for private health insurance plans since the start of open enrollment. Health policy experts now see a space to get to 7 million -- although it's by no means a guarantee.

"October and November were essentially lost months," says Larry Levitt, senior vice president at the Kaiser Family Foundation. "December is the first month where we're getting an indication of how things are working. It's starting to track with what people, particularly the CBO, projected originally."

"It was a very impressive December," says Dan Mendelson, president of health research firm Avalere Health. "The fact that they have about 2 million enrolled is not that far off from 3.3 million."

Avalere did its own projections for health law enrollment, modeling what the numbers would look like if shoppers signed up at the same rate as they did with Part D, the Medicare drug program that launched in 2006. Not accounting for HealthCare.gov's massive technical issues, they had expected 2.4 million people to gain coverage by the Jan. 1 deadline.

In the Part D experience, 34 percent of the first-year enrollees signed up to get coverage at the very first opportunity. Right now, 2 million Obamacare enrollees works out to about 28 percent of the 7 million figure.

"Where they are, with about two million enrolled, if they continue to enroll at the present rate, and there's a little acceleration at the end, they could get to seven million," Mendelson says.

Will enrollment continue apace, or even speed up a bit? That's a bit difficult to predict. Some expect a slight slowdown in the early months of 2014, when shoppers don't face an imminent deadline for purchasing coverage.

"I think January and February will probably slow down, but I would not be surprised at all if March is much bigger than December," Levitt says. "I don't think the outreach campaigns have peaked yet, and March is when you actually have to get coverage. That will be the real story."

And between January and March, there's still space for plenty to go wrong. There will no doubt be lots of stories in early 2014 of how well shoppers do -- or don't -- like the new coverage they purchased through HealthCare.gov. And that could influence potential buyers' decisions of whether they think the new health plans are a worthwhile investment.

"If the stories that start to emerge are like, 'I signed up, I paid, and now I can't get my benefits,' that could be negative," Mendelson says.

Some states, like Oregon and Maryland, are still having lots of technical problems with their own exchanges, and that could depress enrollment in those particular places. Oregon has signed up a paltry 5.9 percent. But Connecticut has already enrolled more people than the federal government projected it would in the entire year.

And, as health policy experts are very quick to point out, hitting 7 million enrollees isn't some magical benchmark, where the Affordable Care Act suddenly has the manpower to work. If the Congressional Budget Office had projected 5 million sign-ups this year -- or 15 million -- we'd have a much different measuring stick. What seems to matter more is the mix of who signs up, whether they tend to be older and sicker -- or include lots of healthy, younger people, too. While the White House has repeatedly promised demographic information, it has not yet released this type of data.

KLIFF NOTES: Top health policy reads from around the Web.

What Obamacare means to the people actually signing up. "Adam Peterson’s life is about to change. For the first time in years, he is planning to do things he could not have imagined. He intends to have surgery to remove his gallbladder, an operation he needs to avoid another trip to the emergency room. And he’s looking forward to running a marathon in mid-January along the California coast without constant anxiety about what might happen if he gets injured. These plans are possible, says Peterson, who turned 50 this year and co-manages a financial services firm in Champaign, Ill., because of a piece of plastic the size of a credit card that arrived in the mail the other day: a health insurance card." Lena H. Sun and Amy Goldstein in The Washington Post.

Patients are cramming in tests before the start of the new year. "Thousands of people are cramming in tests, elective procedures and specialist visits before year's end, seeking out top research hospitals and physician groups that will be left out of some 2014 insurance plans under the new health law, health-care providers say. Ben Rosenthal, a 57-year-old freelance market researcher in Los Angeles, rushed to squeeze in a lung exam the day after Christmas. His new insurance plan under the health law won't cover care at Ronald Reagan UCLA Medical Center in Los Angeles, where he was referred under his current plan." Timothy W. Martin in The Wall Street Journal.

We don't know if Obamacare is working well. But we know it's working. "Healthcare.gov is a (mostly) functioning website. This was no sure thing even a few weeks ago. At the end of November, when officials announced that they had met their goal of constructing a website that worked well for most customers, they were cautious to warn about future problems. Partly that was because their previous predictions of success proved so unbelievably wrong. And partly that was because they feared a late surge of customers would overwhelm the site’s capacity, threatening a whole new period of chaos. But the system held up just fine, as the high enrollment numbers indicate." Jonathan Cohn in the New Republic.