Mortgage markets could come under pressure if the Federal Reserve winds down ongoing purchases of $1.25 trillion in mortgage-backed securities by the end of March as planned, members of the Federal Reserve's Open Market Committee acknowledge, and some are in favor of expanding and extending the program.Minutes of the Open Market Committee's Dec. 15-16 meeting released today show some disagreement over the future of the Fed's MBS purchases, which are widely credited as helping keep interest rates at record lows in 2009.Although the committee last month publicly affirmed its intention to wind down the Fed's MBS purchases by the end of March, it also promised to "continue to evaluate the timing and overall amounts of its purchases of securities in light of the evolving economic outlook a...