Seriously past-due home mortgages continue to decline for the nation’s largest mortgage company, continuing a 15-month path of descent.

Fannie Mae says the share of single-family loans it holds that are 90 or more days past due or in foreclosure fell 8 basis points to 4.19 percent in April, and then dropped another 5 basis points to 4.14 percent in May.

A year earlier, Fannie’s serious delinquency rate was more than a full percentage point higher at 5.15 percent in May 2010. The GSE’s seriously overdue mortgages have been

declining ever since hitting a high point of 5.59 percent in February of last year.

Fannie Mae says servicers completed modifications on 16,419 of its loans in May. For the first five months of the year, loan modifications total 84,133.

Both Fannie Mae and Freddie Mac have announced that they are aligning their servicing guidelines for handling past-due mortgages.

A central goal is to ensure servicers are working with distressed borrowers to find a resolution early in the process so that fewer loans find their way to the serious delinquency stage.

Fannie laid out the specifics of its new guidelines last month, with phased effective dates for each piece of the plan.

According to Fannie Mae’s latest monthly summary report, the company’s total book of business, including its mortgage portfolio and mortgage-backed securities, decreased at an annualized rate of 3.7 percent in May to $3.2 trillion.

Fannie Mae’s gross mortgage portfolio declined at a compound annualized rate of 13.5 percent to $738 billion.