Mortgage guidelines appear to be tightening with the nation’s largest banks.

In its quarterly survey to senior loan officers nationwide, the Federal Reserve uncovered that a small, but growing, portion of its member banks is making mortgage approvals more scarce for “prime” borrowers.

A prime borrower is described as one with a well-documented payment history, high credit scores, and a low monthly debt-to-income ratio.

Of the 53 responding “big banks”, 3 reported that mortgage guidelines “tightened somewhat” last quarter. This is a tick higher as compared to prior quarters in which only 2 banks did.

46 banks reported guidelines unchanged from Q1 2011.

When mortgage guidelines tighten, it adds new hurdles for would-be home buyers. Tighter lending standards means fewer approvals, and that can retard home sales across a region.

Just don’t confuse “tighter standards” with “oppressive standards”.

While it is more difficult to get approved for a purchase home loan in 2011 as compared to 2006, the same basic rules apply:

• Show that you have a history of paying your bills on time
• Show that your income is sufficient to cover your obligations
• Show that you can make a downpayment

And the good news is that, once approved, you’ll benefit from some of lowest mortgage rates in history.

Last week, the average 30-year fixed mortgage was below 4.250% for buyers willing to pay points, and the average 5-year ARM was below 3.000%. The 15-year fixed rate loan was similarly low.

For as long as delinquency rates remain high, expect mortgage guidelines to continue to tighten through the rest of 2011 and into 2012. Therefore, if you’re a “fringe” borrower looking at a purchase in the fall or winter season, consider moving up your time frame. Changing guidelines may render you ineligible for a mortgage.