Merging Cells in Tables in Word

Question

I have a table in Word with 3 rows and 20 columns.  I am trying to take the 1st row and merge the 1st 8 cells into one to make a "header" over the 2nd, 3rd, and 4th columns.  When I execute the merge code below, it merges the columns together, but moves all the other columns to the left.  I want to perform the merge much like you do it in Word where you highlight the cells, select merge and the 3 cells become one leaving the same width.
object startRange = oTable.Rows[1].Cells[2].Range.Start;
object endRange = currentRow.Cells[4].Range.End;
Word.Range mergeRange = aDoc.Range(ref startRange, ref endRange);
mergeRange.Cells.Merge();
Thanks,
Robert