The first wave of Americans to default on their home mortgages appears to be cresting, but a second, far larger one is quickly building.

Homeowners with good credit are falling behind on their payments in growing numbers, even as the problems with mortgages made to people with weak, or subprime, credit are showing their first, tentative signs of leveling off after two years of spiraling defaults.

The percentage of mortgages in arrears in the category of loans one rung above subprime, so-called alternative-A mortgages, quadrupled to 12 percent in April from a year earlier. Delinquencies among prime loans, which account for most of the $12 trillion market, doubled to 2.7 percent in that time.

The mortgage troubles have been exacerbated by an economy that is still struggling. Reports last week showed another drop in home prices, slower-than-expected economic growth and a huge loss at General Motors. On Friday, the Labor Department reported that the unemployment rate in July climbed to a four-year high.

While it is difficult to draw precise parallels among various segments of the mortgage market, the arc of the crisis in subprime loans suggests that the problems in the broader market may not peak for another year or two, analysts said.

Defaults are likely to accelerate because many homeowners' monthly payments are rising rapidly. The higher bills come as home prices continue to decline and banks tighten their lending standards, making it harder for people to refinance loans or sell their homes. Of particular concern are “alt-A” loans, many of which were made to people with good credit scores without proof of their income or assets.

“Subprime was the tip of the iceberg,” said Thomas H. Atteberry, president of First Pacific Advisors, a investment firm in Los Angeles that trades mortgage securities. “Prime will be far bigger in its impact.”

In a conference call with analysts last month, James Dimon, the chairman and chief executive of JPMorgan Chase, said he expected losses on prime loans at his bank to triple in the coming months and described the outlook for them as “terrible.”

Delinquencies on mortgages tend to peak three to five years after loans are made, said Mark Fleming, the chief economist at First American CoreLogic, a research firm. Not surprisingly, subprime loans from 2005 appear closer to the end of defaults than those made in 2007, for which default rates continue to rise steeply.

“We will hit those points in a few years, and that will help in many ways,” Fleming said, referring to the loans made later in the housing boom. “We just have to survive through this part of the cycle.”