The upward climb of mortgage interest rates probably will peak within the next few months and then begin a slow descent, economists and real estate industry leaders predict.

The interest rates on 30-year fixed-rate loans - now a little more than 11 percent - is expected to reach 11.5 percent to 12 percent about midyear before beginning to creep downward.Most predictions, however, put the peak at about 11.5 percent, and several experts warned that any subsequent decline would not be significant.

A half-percentage point increase, from 10.5 percent to 11 percent, adds $38 to the monthly payments on a $100,000, 30-year fixed-rate mortgage.

Many families may have to shelve plans to buy homes as rate increases push payments beyond their reach, but the wait for some will not be long if economists are correct in their expectation that rates will drop slightly below 11 percent by the end of 1989.

All interest rates have been rising over the past two years, responding to Federal Reserve Board actions aimed at cooling off the economy and holding down inflation.

Some housing industry leaders said that although the Fed has been successful, it may not immediately ease its efforts to slow the economy.

"At about midyear, it will become apparent that the economy really is slowing and we'll see the Fed change its policy," said David Berson, vice president and chief economist for the Federal National Mortgage Association (Fannie Mae).

"We're feeling that the Federal Reserve does not need to worry as much about next year," said Warren Lasko, executive vice president of the Mortgage Bankers Association of America.

The homeowners feeling the biggest impact from the increase in interest rates are those with adjustable-rate mortgages. The ARMs have taken such a jump lately that homeowners in at least some parts of the country have gone into what lenders call "payment shock."

The adjustable mortgages typically carry interest rates that change every one, three or five years even though the mortgage itself lasts 30 years.

Typically, the interest rate on a one-year ARM is tied to the yield of one-year U.S. Treasury bonds. Most ARMS have a fluctuation limit, or "cap," of 2 percentage points a year and 6 percentage points over the life of the mortgage.

Two years ago, when fixed-rate mortgages were being offered at 9 percent, one-year ARMs were available for around 7 percent.

Most home buyers decided on fixed-rate loans, but two types of buyers chose ARMs instead: those who didn't intend to stay in their homes more than five years, and those who needed to borrow practically the full price of their home.

Today those borrowers face a double whammy, brought on by two full years of maximum rate increases. Here's how:

Last year, as the Treasury index climbed, so did the one-year ARM rate. A borrower with a $100,000 mortgage who started off paying $1,000 per month, for example, saw his interest rate jump 2 percentage points and his monthly payments to $1,150.

This year the index continued to climb and the one-year ARM rate followed, rising another 2 percentage points, with payments now at $1,300 a month - a jump of 30 percent in only two years.

Anyone considering an ARM these days should take such a worst-case scenario into account, said David Ginsburg, president of Loantech Inc., a mortgage consulting business in Gaithersburg.

"You have to ask yourself, `Can my budget a year from now handle a 9 percent payment? Can it handle an 11 percent payment?'," Ginsburg said. "You need the financial ability and the fortitude psychologically to face the gamble."

Two years ago, California lenders were offering "teaser" rates as low as 6.75 percent - as much as a full percentage point below rates in other urban areas - to attract buyers to the booming West Coast real estate market. When the teaser rates ended, some California borrowers found themselves paying rates that had risen as much as 3 full percentage points.