At a time of grim prospects for Americans without a college degree, nursing can look like a rare chance not just for a job but a real career.

Or at least it did.

There were more than 2.6 million registered nurses employed in the U.S. in 2012, according to the Labor Department, plus another 718,000 licensed practical and vocational nurses and tens of thousands of nurse practitioners and other advanced specialists. RNs made nearly $68,000 per year on average in 2012, and the Labor Department expects employment to grow by more than a quarter between 2010 and 2020.

Nursing’s appeal isn’t just its size or growth prospects but also its low barriers to entry: Registered nurses typically needed only an associate’s degree, and licensed practical nurses don’t need a college degree at all. What do nurses do? And unlike many jobs available to less-educated workers, nursing offers a clear upward path: LPNs can become RNs. RNs can become nurse anesthetists, nurse midwives or nurse practitioners, who can perform many tasks traditionally performed by doctors. Hospitals often provided on-the-job training or tuition reimbursement to help lower-level nurses advance.

But now all of that is changing.

A story in today’s Wall Street Journal looked at the disappearance of many middle-skill jobs in the health-care sector — the kind of jobs that historically have been the entry points into healthcare careers. To dig deeper into the trend, Burning Glass, a Boston-based company that analyzes workforce trends, looked at online postings for healthcare jobs, and nursing in particular, to see how the industry is changing. Their main finding: “The nursing career ladder is becoming harder to climb,” Burning Glass Chief Executive Matt Sigelman wrote in an email.

Among Burning Glass’s findings:

The first rung is harder to reach. Licensed practical and vocational nurses are getting pushed out of hospitals. Burning Glass looked at the 20 biggest acute care hospital organizations. In 2007, such organizations posted one LPN job for every six registered nurse positions. Five years later, the ratio was one to 10.

Who is replacing the LPNs? A combination of more and less-skilled workers. Burning Glass analyzed the skills required for various jobs and found that hospitals are essentially splitting what used to be LPN jobs between nursing/medical assistants and registered nurses. For example, vital signs measurement, once a common task for LPNs in hospitals, is increasingly being performed by RNs instead. Meanwhile less demanding tasks, such as blood-pressure measurement, are being assigned to medical and nursing assistants.

There are still jobs for LPNs, especially in the middle of the country, but the postings are mostly in nursing homes, doctor’s offices and long-term care facilities. Such jobs are less prestigious and lower paying than hospital jobs, and are less likely to offer opportunities for advancement.

The higher rungs are harder to reach. Even a registered nursing job no longer offers a sure-fire career path. Hospitals are increasingly requiring nurses to have a bachelor’s degree, relegating many nurses with associate’s degrees to jobs outside of hospitals.

Mr. Sigelman said there is evidence that the nursing profession is bifurcating between hospital-based jobs with clear opportunities for advancement and office jobs that offer more limited options. Burning Glass found that RN job postings calling for hospital experience have risen 55% since 2010, even as an increasing number of nursing jobs are outside of hospitals. That means it’s getting harder to find a hospital job, and harder to advance without one.

The growth is at the bottom and the top. There are still plenty of entry-level healthcare jobs out there. Postings for medical assistants, for example, are up 57% since 2007, and openings for home health aides are up 43%. Many nursing homes, in fact, are struggling to fill low-level jobs.

The problem is, those jobs offer little opportunity for advancement. Nursing aides can’t readily become registered nurses without significantly more education. And many of them, such as records clerks and pharmacy technicians, may be vulnerable: Mr. Sigelman notes that postings for medical transcriptionists, once a major source of low-level jobs, are down 52% since 2007, as voice-recognition software and electronic medical records eliminate the need for human transcriptionists. Low-skill jobs, Mr. Sigelman says, “often live at the precipice of obsolescence.”

For nurses who do manage to get on the ladder, however, there’s plenty of room to climb. Demand is booming for nurses with advanced degrees or specializations. Postings for critical care nurses are up 27% since 2010, Burning Glass found, and openings for intensive care nurses are up 17%. Such jobs can pay extremely well — nurse anesthetists make more than $150,000 per year on average, according to a recent Census report.

“It’s a bad news-good news thing,” Mr. Sigelman says. “The bad news is it’s harder to get on the ladder, but the good news is that there’s a ladder and there is a pathway up.”