Membership in the nation’s top health insurance plans fell by 1.3% in the third quarter of 2009, as the recession and high unemployment rates battered enrollments, according to a new report.

The aggregate decline of 1.7 million members in the third quarter, compared to the same period in 2008, occurred in both the fully insured and Administrative Services Only (ASO) segments, according to research by Mark Farrah Associates, a Kennebunk, Maine-based provider of health market data. The data is the latest available from the health plans.

In the third quarter of 2009, the eight leading U.S. health insurers reported a total of 131.5 million members. ASO enrollment decreased 1.1% and risk enrollment declined 1.6% for these plans between the third quarter of the two years.

When comparing the second quarter of last year to the third, the decline was 860,000 members, while ASO enrollment fell by 0.4% and risk enrollment dropped 0.9%.

Health Care Services Corporation, which includes Blue Cross and Blue Shield plans in Illinois, New Mexico, Oklahoma, and Texas, and Humana were the only organizations among the top eight plans to see enrollment gains in the third quarter, researchers said. Humana was also among the few plans that saw year-to-date profit margin improvement when compared to 2008, according to Mark Farrah Associates.

The eight top health insurers include Aetna, CIGNA, Health Care Service Corporation (HCSC), Health Net, Humana, Kaiser Permanente, UnitedHealth Group and WellPoint. Collectively, they cover nearly 60% of the total insured population in the U.S., which reflects a 0.1% increase from the third quarter of 2008.