* Expects 13-16 pct 2018 earnings growth
* Sees rise in insurance customers, Optum growth
* Shares up more than 5 percent after forecast (Adds details on UNH association plans, background on Obamacare)
By Caroline Humer
Oct 17 (Reuters) - UnitedHealth Group Inc, the largest U.S. health insurer, said earnings will grow 13 to 16 percent in 2018 as medical costs remain low, and that it expects to benefit from new insurance products backed by U.S. President Donald Trump.
UnitedHealth, the industry bellwether, is the first health insurer to report and its third-quarter net earnings rose 26.3 percent, beating analysts' expectations. Its shares rose more than 5 percent, also lifting rival insurers such as Anthem Inc , Aetna Inc, and Cigna Corp.
Trump, who promised to repeal predecessor Barack Obama's signature the Affordable Care Act - so-called Obamacare - but failed to do so, last week signed an executive order to weaken it.
UnitedHealth sold Obamacare individual plans in about two dozen states before booking financial losses and then mostly pulling out of the market this year.
UnitedHealth's new Chief Executive Dave Wichmann now sees opportunities in the plans outlined by Trump.
Those plans envision the creation of more associations that will allow people to band together to buy cheaper, less regulated health plans, and extends the duration of benefit-light short-term plans to one year from three months.
Wichmann said the company would work with the administration on the new products it had outlined, and that it already has 300,000 customers in association health plans.
"We have a great deal of experience in the areas covered in the first order -- short-term policies, association plans and expanded use of HRAs," Wichmann said on a conference call, referring to taxfree employer-funded health reimbursement accounts.
While it could take until 2019 until association plans are for sale, the short-term plan extension may come sooner.
Wichmann also said the impact of Trump's decision to scrap about $7 billion in subsidies paid to health insurance companies for low-income patients would be "extremely small". The company only has about 30,000 customers who are eligible for these subsidies in four states.
Boosting growth next year, the company also expects to increase the number of insurance customers and grow its Optum pharmacy and data businesses, adding to the benefits of its medical costs remaining within a targeted range.
Wichmann said the 2018 outlook takes into account the reinstatement of Obamacare's 3 percent tax on health insurance, putting the cost at about 75 cents in earnings per share growth. The tax was suspended in 2017 and will be back in effect after Republicans failed to repeal the Affordable Care Act.
UnitedHealth said net earnings rose 26.3 percent to $2.49 billion, or $2.51 per share, in the third quarter ended Sept. 30. Excluding items, the company earned $2.66 per share, beating the average analyst estimate of $2.56 per share, according to Thomson Reuters I/B/E/S.
Leerink analyst Ana Gupte said investors had been concerned that the Wall Street consensus was too high and she was now hopeful that profits could rise even further. "Usually they are a conservative company," Gupte said.
The insurer said its withdrawal from individual insurance markets, combined with a health insurance tax deferral, reduced third-quarter revenue by about $1.6 billion and lowered the revenue growth rate by 4 percent.
UnitedHealth shares were trading at $204.28, up $11.08, or 5.8 percent. (Reporting by Caroline Humer in New York and additional reporting by Divya Grover in Bengaluru; Editing by Maju Samuel and Susan Thomas)