Detecting pitch values for singing voice in the presence of music accompaniment is challenging but useful for many applications. We propose a trend estimation algorithm to detect the pitch ranges of a singing voice in each time frame. The detected trend substantially reduces the difficulty of singing pitch detection by reducing a large number of wrong pitch candidates either produced by musical instruments or the overtones of the singing voice. The proposed algorithm can be applied to improve the performance of singing pitch detection. Quantitative evaluations show that proposed trend estimation improves an existing algorithm significantly. The results from the MIREX 2010 competition show that our system achieves the best overall raw-pitch accuracy for vocal songs.