NEW YORK  — Amazon’s profits were hit or miss for its first couple of decades. But now, for the first time, they are starting to look consistent — with four consecutive quarters above $1 billion in profit.On Thursday, the company reported a net income of $2.9 billion in the third quarter, surpassing analyst expectations. But the focus on profit that investors have long sought was not enough to keep them happy. Shares dropped more than 7 percent in aftermarket trading, after Amazon also announced that revenue in its core retail business was growing more slowly than in the past.

The company’s sales were up 29 percent to $56.6 billion, below what analysts had predicted. In Amazon’s biggest business, its online store, sales rose just 11 percent over the past year. A year ago, the pace was twice as quick.

“They are saturating the core market in the U.S. for e-commerce,” said Cooper Smith, who leads Amazon research for Gartner L2. “This is a company in transition,” moving from a focus on acquiring more customers to getting more growth from people who already shop the site.

Profit was up in part because Amazon’s leadership has focused on trimming costs to counteract the slower growth. “We have really been able to cut back in a number of key areas,” Brian Olsavsky, the company’s chief financial officer, said in a call with reporters.

He said the company saved money by redeploying engineers rather than hiring more new staff and being more efficient in its warehouses and data centers. About 613,000 people now work at Amazon, up 13 percent from the same time last year.

“The theme will be banking some of the investment from prior years,” Olsavsky said in a later call with analysts.

Amazon has made a push in recent months to get more out of its high-margin areas, especially its industry-leading cloud computing services, a huge business with 31 percent operating margins this quarter.

Its Prime memberships are another area: This year, Amazon raised the annual price of a Prime Membership by 20 percent, to $119. The number of new members signing up for Prime has been slowing, down to just 1 to 2 percent a quarter, according to Consumer Intelligence Research Partners, a firm that surveys users for brands and investors.

The company is also focused more on its advertising business. Amazon’s website has had advertising such as Sponsored Products for years, but ads have recently become the company’s fastest growing business. In September, it became the third-largest digital advertising service in the United States, behind just Facebook and Google, according to estimates from eMarketer. Ads provided a bright spot in the earnings, providing about $2.5 billion in revenue in their third quarter of triple-digit growth.

Those lucrative markets let Amazon plow cash into new businesses that require intense physical investments and engineering talent, like expanding fast grocery delivery or building out its Alexa ecosystem. “Now the investment is not a cycle — it’s just a way of life,” said Brian Nowak, who researches Amazon at Morgan Stanley.

The company is also building out same-day delivery, which drives sales but is expensive to offer, and offering new video content. And it has expanded into new markets, like pharmacies with its $1 billion PillPak acquisition this summer, and groceries with its Whole Foods acquisition. Amazon now offers Prime members free two-hour delivery from Whole Foods in more than 60 cities, reaching more than 17 percent of the American population, according to Morgan Stanley estimates.

Early this month, Amazon announced it was raising the minimum wage for its warehouse and customer service workers to $15 an hour. The move, which Amazon said could affect more than 250,000 Amazon employees and more than 100,000 seasonal workers, came as the company started to look to attract workers for its holiday peak.

As part of the wage increase, Amazon ended the bonus and stock grants it had been giving workers, which analysts estimated would offset some of the costs, as would increased efficiencies using robotics.