Federal Reserve Gov. Martha Seger predicted yesterday that the money supply is growing slowly enough that all of its various measures will be back within their target ranges "probably by late March or for sure in April."

Slower money growth plus recent indications that the economy may be expanding less rapidly than expected give the Fed some room to maneuver, Seger said. The current risks facing the economy are more that it will grow more slowly than it should rather than that it will expand too rapidly and make inflation worse, she indicated.

Seger's remarks were made during a confirmation hearing before the Senate Banking Committee. They came one day after a meeting of the Fed's policy-making group, the Federal Open Market Committee, or FOMC, at which it set a policy course for the next two months.

Very rapid growth of the money supply during the past several months has pushed the monetary aggregates to or above the upper limits of the target ranges set for them by the FOMC. Some financial analysts had speculated that the FOMC might decide at this week's meeting to tighten its policy stance somewhat -- that is, seek or allow short-term interest rates to rise -- to slow down the growth of money.

But the analysts generally decided that such a tightening was unlikely both because of the evidence that money growth was already slowing and because of the statistics suggesting that the economy may be expanding at a slower pace than the Fed thinks is desireable.

Seger's remarks imply that the FOMC also expects slower money growth in coming weeks, and therefore that it is unlikely that the committee would have decided to tighten policy this week.

President Reagan nominated Seger to become one of the seven Fed governors last year. Her nomination was opposed by the Democratic members of the Banking Committee last year. The nomination was sent to the full Senate on a straight party-line vote with the committee's Republican majority supporting her. However, because of the Democratic opposition, the nomination never came to a vote and Reagan later gave her a recess appointment after Congress adjourned.

Banking Committee Chairman Jake Garn (R-Utah) said he expects the committee to vote on her nomination again within a few days and that it again would be approved on a straight party line basis.

In her testimony, Seger expressed serious concern about the problems of thrift institutions across the country, which could be worsened by rising interest rates. Rising rates could also have a serious effect on the ability of less developed nations to continue payments on their international debts, she said.

Seger also said that the size of the federal budget deficit is causing the Fed to follow a tighter policy course than it would with a smaller deficit.

Meanwhile, Fed Chairman Paul A. Volcker again urged Congress to reform the nation's complex banking regulatory system but warned that taking away any of the Fed's supervisory roles could hamper its execution of monetary policy and make it more difficult for the central bank to play its role as a "lender of last resort."

Volcker said he supports an administration study that calls for streamlining the numerous agencies that oversee banks, thrifts and other financial institutions in the interest of simplification and greater coherence in a time of rapidly changing technology.

But, he told the House subcommittee on commerce, consumer and monetary affairs, the Federal Reserve must be able to retain its "hands on" bank-examining authority and direct supervisory role to respond quickly to emergencies like the recent run on Ohio savings and loans and the liquidity problems experienced by the Penn Square and Continental Illinois banks.

"There's no substitute for having your own people there rather than relying on examiners from other agencies," Volcker said.

"It seems like the Fed's saying that it's not ready to cut the apron strings," replied Rep. John Spratt (D-S.C.).

"They're more than apron strings," Volcker responded. "We're saying there's an umbilical cord between banking regulations and monetary policy. If you cut the umbilical cord, you're going . . . to have an abortion."

However, he said the central bank would be happy to give up its marginal oversight responsibilities such as enforcing the truth-in-lending laws.