Fannie Mae’s book of mortgage business decreased at a compound annualized rate of 4.8%, compared to 4% last month, according to the GSE’s monthly summary.

For the full year of 2012, Fannie Mae’s gross mortgage portfolio totaled about $633 billion, down from $708 billion a year prior. 

The conventional single-family, serious delinquency rate fell one basis point to 3.29% in December, remaining relatively unchanged from 3.3% in November. 

The multifamily seriously delinquency rate also fell one basis point to 0.24% in December, compared to 0.25% in November. 

Fannie Mae completed 12,095 loan modifications, totaling 163,412 for 2012. Last month, the government-sponsored enterprise completed 151,317 loan modifications. 

In December, the GSE’s gross mortgage portfolio grew at a compound annualized rate of 1%.

The monthly report summary contains information about Fannie Mae’s monthly and year-to-date activities including mortgage-backed securities, interest-rate risk measures and serious delinquency rates.