RIO VISTA, Calif. (AP) _ A 45-ton humpback whale who prefers warm inland waters over the cool Pacific Ocean may be pregnant, rescue workers said Wednesday.

The theory surfaced as experts regrouped to find an effective way to coax the whale out of the Sacramento River and back to sea, and as offers of contributions were reported flowing in from major corporations, singer Wayne Newton and Hollywood actors.

''Based on the behavior pattern, it's a strong possibility (that the whale is pregnant),'' said Bev Passerello, spokeswoman for the state Office of Emergency Services.

It would help explain the 40-foot whale's rugged, maybe maternal, resistance to leave the warm waters where it wandered Oct. 11, she said.

Humphrey, as the whale was nicknamed before the latest theory surfaced, may be looking for comfortable water to give birth in, much like female whales' migration to the coastal waters off Baja California and the Hawaiian islands, she added.

''The whale has consistently resisted any efforts to force it, and I think we should refer to it as 'it' at this point, to force it back to the ocean,'' she said.

''We're trying to get some people together on this now,'' she said, adding that National Marine Fisheries Service wildlife biologist is checking research literature for additional information to try to verify the speculation.

If the whale, a member of an endangered species, is pregnant, nothing else will be done to try to herd it back under the Golden Gate Bridge, she said.

National Fisheries conservation chief Jim Slawson took the news with skepticism. ''The last we knew it was male, and we still believe it's a male,'' he said Wednesday.

The original decision that Humphrey was male was based largely on tail markings common to males and ''rare'' among females, according to Jay Ziegler, a legislative aide to state Sen. John Garamendi.

''It seems to be carving out a micro-territory, which all animals do when they're getting ready to give birth,'' Ziegler said.

On the other hand, he said, the normally gregarious humpback is surrounded by other whales when it prepares for calving.

A whale expert hired by the National Fisheries to try to tag Humphrey with a space age tracking device reported the beast's size led him to think the whale is a juvenile and thus incapable of being pregnant, whatever its sex, Slawson said.

Current efforts to determine if the whale is male or female include checking photos taken of it and trying to match them with those taken of humpbacks by researchers earlier, he said.

Otherwise, Slawson said, ''It (Humphrey) would have to be rolled over and (have) its belly examined.''

The windy and choppy weather in the delta severely limits underwater visibility, preventing any attempt by divers to swim under the whale and resolve the question, he said.

A meeting in Sacramento of state and national agencies was set for Thursday. Rescue efforts probably will be in limbo until Friday, said Slawson.

Meanwhile, California Marine Mammal Center executive director Peigin Barrett said an interest-bearing money market account dubbed ''Humphrey's Fund'' has been set up to collect contributions to help pay for the rescue project.

''I was on the phone all day just calling corporations,'' she said, adding that offers have been made by Chevron, American Airlines, Union Oil and Pacific Gas and Electric Co.

Also volunteering to help have been Las Vegas singer Wayne Newton and ''Actors and Others For Animals'' in Hollywood, she said.