Fannie Mae's portfolio of home loans and bonds grew at a 2.3% annual rate last month from August, to $761.4 billion, as federal regulators seized control of the government-sponsored enterprise.
The portfolio had grown at an annual rate of 3% in August, Fannie said Thursday.
A day earlier it said it may write down at least $20 billion of deferred tax credits, potentially cutting its book value in half and increasing the likelihood of a cash injection from the Treasury Department.
Last week Freddie Mac said its portfolio contracted 3.2% last month, to $737 billion, after contracting 4.7% the previous month.
Regulators took over both Fannie and Freddie last month in response to concerns that further losses would limit their mortgage bond purchases and drive up loan rates. Before the takeover they had lost $14.9 billion over the previous four quarters.
A jump in borrowing costs this month is making it harder for Fannie and Freddie to make purchases.
Fannie and Freddie "demand remains weak as mortgages continue to trade tighter" than their corporate debt, Matthew Jozoff, a mortgage bond analyst at JPMorgan Chase & Co. in New York, wrote in a research report issued Tuesday.
Fannie said the August "serious delinquency'' rate in the single-family loans it either owns or guarantees within bonds more than doubled from a year earlier and rose 12 basis points from July, to 1.57%.