Fannie Mae's gross mortgage portfolio dropped to a compound annualized rate of 21.4% in January, according the government-sponsored enterprise's monthly summary.

For the full year of 2012, the GSE's gross mortgage portfolio totaled about $633 billion, down from $708 billion a year prior, the firm noted in its December summary. 

The book of mortgage business continued to decline at a compound annualized rate of 1.8% last month, up from 4.8% in December. 

The conventional single-family, serious delinquency rate also fell 11 basis points to 3.18% in January. 

However, the multifamily serious delinquency rate rose 11 basis points to 0.35% in January. 

Fannie Mae completed 14,923 loan modifications in January, up from 12,095 in December.