In a new sign that implementing the health law could take longer than expected, insurer Aetna said Tuesday it lowered the number of medical policies it expects to sell through online marketplaces that open for business in October.

“This is going to be a slow uptake,” Aetna CEO Mark Bertolini told investment analysts on a call to discuss financial results. “The process required to sign up, to get the subsidies, is going to take some time. And I think this is a two-year ramp to get the individual exchanges up to a level where customers are going to feel appropriate signing up. And so our estimates of what we believe … enrollment [will be] are dropping for the first year.”

He didn’t give a number, and insurers rarely disclose projections for specific business lines. But Aetna offered nothing to challenge perceptions that it will approach the Affordable Care Act’s subsidized marketplaces, also known as exchanges, with great deliberation.

Without naming specific states, the company cut from 15 to 14 the number of states in which it might sell exchange plans to individuals. Aetna might even withdraw at the last minute if exchanges aren’t ready or look unprofitable, Bertolini said. Under the ACA’s requirement that everybody buy health insurance or pay penalties, consumers without coverage from employers or government programs such as Medicare are supposed to start shopping for exchange plans on Oct. 1.

“We’re not going to go in for a land grab,” Bertolini said. “Obviously at the end of all this we have an opportunity to pull out in September. And we continue to hold that as an option should the exchanges not develop favorably or they ask for unreasonable rates.”

The caution of No. 3 health insurer Aetna echoes that of No. 1 UnitedHealthcare, which has said it will be “very selective” in selling exchange plans.

Aetna itself is helping hold down exchange enrollment by offering to renew policies of existing customers before the end of this year, thus delaying potential price increases associated with the health law. Rating rules expected to raise premiums for younger, healthier customers kick in Jan. 1, but only for plans starting a new policy year. Approving a one-year renewal before the calendar turns over could delay the price hike for most of 2014 and prove to be more attractive for some clients than buying an exchange policy.

“We are going to offer that opportunity as part of our renewal strategy with accounts,” said Bertolini.

Like other insurers, Aetna is attempting to compete on exchanges with narrower networks of hospitals and doctors who agree to lower prices in return for more new customers. The insurer has signed deals with two-thirds of the care providers it hopes to include, Bertolini said, adding that Aetna’s exchange networks are a fourth to half the size of its regular provider organizations. The narrower the network, the closer Aetna’s costs will be to lower rates paid by Medicare rather than higher commercial reimbursement, he said.

Challenges to opening the health marketplaces include building complex computer systems; persuading insurers to participate; running federally managed exchanges in hostile Republican states; relying on limited budgets to educate largely ignorant consumers; and enrolling young, healthy members to finance the expenses of those with pre-existing illness who will be first in line to join. Health and Human Services Secretary Kathleen Sebelius says the exchanges will be ready.