KUALA LUMPUR (Reuters) -Malaysia's economy expanded at its fastest rate in 18 months in the second quarter helped by stronger household spending, exports and investment, with the central bank forecasting full-year growth to come in near the upper end of its forecast range. Gross domestic product grew 5.9% in the April-to-June period, data showed on Friday, accelerating from 4.2% in the first quarter and surpassing analysts' and early government estimates for a 5.8% rise. On a quarter-on-quarter seasonally adjusted basis, GDP rose 2.9%, compared with a 1.5% rise in the January-to-March period, data from Bank Negara Malaysia (BNM) and the Statistics Department showed.