Home prices in the nation’s 20 biggest cities dipped 0.1 percent in October from the prior month but rose strongly from the same month last year, according to a closely watched index, indicating the start of the seasonal slowdown in home prices.

It was the first month-over-month dip after six months of gains for the Standard & Poor’s/Case-Shiller index of 20 large cities. The index was up 4.3 percent compared with October 2011.

“Looking over this report, and considering other data on housing starts and sales, it is clear that the housing recovery is gathering strength,” said David M. Blitzer, chairman of the index committee at S&P Dow Jones Indices. “Higher year-over-year price gains plus strong performances in the Southwest and California, regions that suffered during the housing bust, confirm that housing is now contributing to the economy.”

Five cities in the 20-city index turned negative, posting month-over-month declines in October after being in positive territory the month before. Those five cities were Atlanta, Dallas, Miami, Minneapolis and Seattle. Among all of the cities, Chicago posted the weakest numbers, dropping 1.5 percent, followed by Boston, where prices fell 1.4 percent.

Las Vegas posted the strongest gain, with prices up 2.8 percent.