The Department of Health and Human Services on Tuesday declared a "significant milestone" for President Obama's health care law by claiming that 4 million Americans had now signed up for coverage. But a closer examination of the numbers suggests that the pace of sign-ups is slowing.

In a similar blog post on Jan. 24, Marilyn Tavenner, administrator of the Centers for Medicare & Medicaid Services, praised the "milestone" of 3 million sign-ups. That represented a gain of 800,000 individuals since the end of December, when HHS had put the number at 2.2 million.

But the Tuesday claim of 4 million sign-ups represents a smaller 700,000 gain from the 3.3 million total the administration reported as of Feb. 1.

Last year, before the law's exchanges had opened, an HHS memo had projected about 200,000 more individuals would sign up in the month than in January. That memo estimated nearly 1.3 million sign-ups in February -- or nearly 600,000 more than HHS claims have signed up thus far.

The open enrollment period for health insurance extends through the end of March, and the administration is hoping for a late surge in sign-ups, especially among young and healthy individuals. Before the botched rollout of the law's exchanges, the administration had defined success as 7 million total sign-ups.

The HHS numbers include only those who have selected a plan through one of the exchanges, rather than the number who have paid -- which is typically how insurers measure enrollment. As a gauge, last week California reported that 20 percent of those who signed up in the state as of Jan. 31 hadn't paid first month's premiums.

Additionally, HHS does not reveal how many of those signing up for insurance through the exchanges were previously covered. Millions of Americans received insurance cancellation letters last fall, as insurers were forced to discontinue policies that didn't live up to Obamacare's guidelines.

HHS is expected to release final total sign-up numbers for February by the middle of next month.