Fannie Mae’s (FNM) book of business grew at an annualized rate of 5.2% in September to $3.24trn at the end of the month, according to its monthly summary. Even though Fannie’s issuance fell for the third straight month and the delinquency rate continued to climb, its total mortgage portfolio increased in September. Fannie, which purchases and securitizes mortgage loans, registered $59.2bn in issuance during September, a 4.6% drop from $62.1bn in August but almost double $38.3bn in September 2008. The gross mortgage portfolio grew 22.4% in September, up to $792.6bn from $779.6bn in August. It’s also a 0.9% increase fro $761.3bn in September of last year. The rate of seriously delinquent conventional single-family mortgages jumped 28bps in August to 4.45%, compared to 1.57% in August 2008. Multifamily delinquencies held steady in August, matching July’s 0.56% rate. Write to Jon Prior.