The BSE benchmark Sensex fell by over31 points in the opening trade today as investors preferred to adopt a cautious approach ahead of the vote on FDI inParliament and trimmed their positions.The 30-share index, which had lost 34.58 points in the previous session, fell by 31.29 points, or 0.16 per cent, to 19,274.03 in first five minutes of trading.Similarly, the wide-based National Stock Exchange Nifty Index shed 8.70 points, or 0.14 per cent, to 5,862.25.Further, a weakening trend on the other Asian markets in line with overnight losses on the US markets after poor datain the manufacturing sector in November triggered selling, they said.Meanwhile, in the Asian region, Japan's Nikkei was down by 0.33 per cent, while Hong Kong's Hang Seng index shed 0.14 per cent. The US Dow Jones Industrial Average ended 0.46 per cent lower in yesterday's trade.