B RYAN AND FOLA Nelson were excit- ed about their upcoming five-night Bahamas cruise on the Carnival Fascination. It was to be their last vacation before the birth of their first child.

Then, not long before their departure, Carnival delivered some bad news: Not only would Fola Nelson be denied boarding, but the cruise line would pocket her entire fare minus port taxes.

Why? Because like many other cruise lines, Carnival bans passengers who are 25 weeks or more pregnant.

"My wife will be 10 days over that," says Bryan Nelson, a teacher in Minneapolis. "And despite her doctor's OK, the cruise line is sticking to its policy."

Cruise lines' rules on pregnancy are a common source of complaint from travelers. But like so many other cruise industry policies, this one wasn't always a hard-and-fast rule. Had Nelson become pregnant a decade ago, the company probably would have let her reschedule at minimal cost.

Not today. And the change is something that her cruise line seems happy to let the world know about.

Carnival's policy allows pregnant women to sail only through the 24th week of pregnancy. Every passenger who is expecting must show a physician's letter verifying that mother and baby are in good health and fit to travel. The letter must also include estimated date of delivery.

"Carnival's pregnancy guidelines are put in place as a precaution to protect the unborn baby and the mother," says Aly Bello, a spokeswoman for the cruise line.

That makes sense. Cruise ships offer reasonable emergency medical facilities for guests and crew members. But prenatal and early infant care can require specialized diagnostic facilities or treatment that might not be available on a ship or in the nearest port of call.

Even with the rules in place, complications can arise. This month, a 31-year-old passenger was airlifted from the Disney Magic, 180 miles off the Texas coast, because of medical problems related to her pregnancy.

Other companies have virtually identical policies. Norwegian Cruise Lines refuses to admit passengers past the 24-week mark. So does Royal Caribbean.

"This decision is made because of the unique nature of a cruise ship being at sea for extended periods of time and the possibility of a guest's medical condition becoming critical during those times at sea," says Royal Caribbean spokeswoman Cynthia Martinez.