The number of residential properties in negative equity edged up slightly in the first quarter in Greater Des Moines, according to real estate analytics firm CoreLogic Inc. Mortgages in negative equity, meaning borrowers owe more than the properties are worth, rose to 14,906, or 13.3 percent of all residential properties with a mortgage, from 14,788 in the fourth quarter of 2012, CoreLogic said today. The national aggregate value of negative equity decreased more than $50 billion to $580 billion at the end of the first quarter from $631 billion at the end of the fourth quarter of 2012, driven in large part by an improvement in home prices, according to a news release. Negative equity, also referred to as being upside down or underwater on a loan, can occur because of a decline in value, an increase in mortgage debt or a combination of both.