NEW YORK (Reuters) - Defaults on privately insured US mortgages rose sharply in April, showing homeowners continue to struggle to keep up with loan payments.

But a one-time change in some of the data may have painted a worse picture than reality, according to Mortgage Insurance Cos. of America (MICA), which compiles the data from information provided by six of the seven largest US mortgage insurance providers.

MICA said the numbers showed 73,880 insured borrowers were at least 60 days late on payments in April. That is up from 43,161 a year earlier.

But the increase includes both newly reported defaults for the month, as well as previously unreported defaults by one of the major lenders, said MICA.

The trade group said it had no way of adjusting the numbers to account for the change in data reporting.

"While the change in reporting methodology by a major lender has resulted in an increase in reported delinquencies, it is important to note that this is a one-time adjustment," MICA's executive vice-president Suzanne Hutchinson said in a statement.

"Overall, the market is returning to fundamentals," she added, noting there was an 11.7 percent increase in new insurance written in the month, reflecting a "return to quality in the marketplace."

MICA compiles its data from information provided by American International Group Inc.'s United Guaranty Corp., Genworth Financial Inc., MGIC Investment Corp., Old Republic International Corp., PMI Group Inc. and Triad Guaranty Inc.

It declined to say which lender had changed the way it reported the numbers. The number of mortgage holders who are late on payments is key because this is often a precursor to foreclosure.

Private mortgage insurance lets people buy homes with down payments of less than 20 percent and guarantees lenders will be repaid even if borrowers default.

Amid the sub-prime mortgage crisis, lenders nationwide have been tightening underwriting standards, forcing prospective homeowners either to put more money down, to find new means to borrow, to buy less costly homes, or defer purchases altogether.

Rising defaults have led at least one US mortgage insurer, Genworth Financial, to step up its work with loan servicing companies to increase "workouts" for troubled borrowers to reduce the number of claims. Workouts can lower monthly payments or modify loan terms.

Another, Radian Group Inc., a large mortgage insurer that is not included in the MICA statistics, has said it will no longer insure home loans if borrowers cannot document income or assets, citing the "poor performance" of such loans. The loans are often known as "liar loans" because they allow borrowers to overstate their financial health.

The number of traditional mortgage insurance policies issued in April was 108,322, down nearly 27 percent from a year earlier, Washington, DC-based MICA said. But the amount written was $19.45 billion, up 11.8 percent from a year earlier.