Noting several factors, including steady membership and premium revenue growth, A.M. Best affirmed the ratings of the majority of the insurance and HMO subsidiaries of Aetna Inc.

The ratings service affirmed a financial strength rating of A (Excellent) and issuer credit rating of “a+” to the Aetna Life Insurance Co., Aetna Health Insurance Co. of New York and Aetna Health Inc., a New York corporation.

A.M. Best also affirmed a financial strength rating of A- (Excellent) and issuer credit rating of “a-” to the Aetna Health Insurance Co., and an issuer credit rating of “bbb-” to Aetna Inc.

The outlook for all ratings is “stable,” according to A.M. Best.

The ratings service found the subsidiaries to have positive operating performance, good liquidity and solid risk-based capitalization levels, accounting for their ratings.

Aetna’s operating performance has been positive due to stable medical trends and significant improvements in its administrative cost ratio, A.M. Best noted, however, operating earnings are expected to be pressured going forward due to medical expense increases and lower incremental improvements in the administrative expense ratio.

Earnings will also be negatively affected by the increase in GAAP pension expense Aetna will incur in 2009, the ratings service said.

Offsetting rating factors include a concentration of business in the self-funded commercial market, lower operating margins and pressure on medical expenses, A.M. Best said.