* Expects 13-16 pct 2018 earnings growth
* Sees rise in insurance customers, Optum growth
* Shares up 5 percent after forecast (Adds details from conference call)
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, even as it adjusts to new healthcare rules that U.S. President Donald Trump has promised to put in place.
UnitedHealth, the bellwether for the industry, is the first health insurer to report and its third-quarter earnings on Tuesday beat analysts' expectations. Its shares rose more than 5 percent, pulling rival insurers such as Anthem Inc, Aetna Inc, and Cigna Corp higher as well.
Trump last week took measures to weaken predecessor Barack Obama's signature healthcare law, known as Obamacare. He said he would create new health insurance products and cut off about $7 billion in subsidies paid to health insurance companies for low-income patients.
After largely pulling out of individual insurance exchanges this year, UnitedHealth's new Chief Executive Officer Dave Wichmann said the company only had about 30,000 customers receiving subsidies in four states and that he expected any impact to be "extremely small."
Wichmann said the company would work with the administration on the new products it outlined in the executive order.
"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," he said on a conference call, referring to taxfree employer-funded health reimbursement accounts.
Besides the boost to growth due to medical costs remaining within a targeted range, the company also expects to keep and increase insurance customers and grow its Optum pharmacy and data businesses.
Wichmann said the outlook took 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.
UnitedHealth's total revenue rose 8.7 percent to $50.32 billion, narrowly missing analysts' estimate of $50.35 billion.
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 $203.29, up $10.09, or 5.3 percent. (Reporting by Caroline Humer in New York and Divya Grover in Bengaluru; Editing by Maju Samuel and Susan Thomas)