DUBAI, April 10 (Reuters) - Saudi International Petrochemical Co (Sipchem) missed analysts' forecasts by a wide margin on Thursday after unplanned shutdowns limited its first-quarter net profit increase to 6.5 percent. Sipchem made a profit of 68.7 million Saudi riyals ($18.3 million) in the three months to March 31, up from 64.5 million riyals in the prior-year period, according to a bourse filing. Four analysts polled by Reuters on average forecast Sipchem's quarterly profit would be 159.4 million riyals. ...