Both Fannie Mae and Freddie Mac held onto their ample shares of mortgage-backed securities with a slight bump during the first quarter of 2013, according to a new Inside The GSEs analysis. The two GSEs issued a combined $355.8 billion in MBS during the first quarter, a 0.9 percent increase from the fourth quarter of 2012. Compared to the first three months of last year, Fannie and Freddie saw a 16.6 percent increase in MBS issuance. Between the two companies, Fannie and Freddie registered an abundant 76.0 percent share of new MBS during the period that ended March 31, 2013, up from 75.8 percent the two companies held during the fourth quarter of 2012 but lower than the 78.0 percent both GSEs held during the first quarter of 2012.