PREVIEW-Oct ISM services index seen rising to 51.5

* What: Institute for Supply Management Oct services index

* When: Wednesday, November 4, 10:00 a.m. (1500 GMT)

* Economists' median forecast: 51.5

NEW YORK, Nov 4 (Reuters) - The U.S. services sector is thought to have grown for a second consecutive month in October, according to a Reuters' poll of economists.

The Institute for Supply Management's services index is estimated to have risen to 51.5 in October from 50.9 in September, according to a median forecast culled from the Reuters poll.

A reading above 50 indicates growth in the non-manufacturing sector.

If the index comes in as forecast, it would mark the highest level since April 2008, when the index came in at 51.9. The index reached a recent trough of 37.4 in November 2008.

Estimates from the 68 economists polled ranged from a low of 49.2 to a high of 54.4.

The Institute for Supply Management will release the October services index at 10 a.m. (1500 GMT) on Wednesday. (Reporting by Chris Reese; polling by by Bangalore Polling Unit; Editing by Andrea Ricci)