One month does not a trend make, but early indicators hint that mortgage lenders may be stretching the credit box to boost volume. Fannie Mae and Freddie Mac issued $91.71 billion of single-family mortgage-backed securities in January, a stiff 18.3 percent drop from the previous month. Some of the decline was the result of Fannie’s heavy volume in December, when its business flow was catching up from an unusually slow November. But mortgage interest rates have edged up, and there is widespread expectation in the industry that refinance volume in 2017 will drop sharply from last year. Higher rates could also tarnish a housing market that’s bumping up against affordability and supply issues.