After slugging away for years in the unglamorous ranks of amateur women's boxing, Marlen Esparza was thrilled when her sport finally became an Olympic one.

But the invitation to London came with a catch: The petite, 106-pound national champion from Houston would have to gain six pounds to compete against bigger, stronger women if she wanted to qualify.

"It was another mountain to climb," said Esparza, who fights Tuesday night to try to win a spot on the three-member U.S. team. "I already thought it was going to be tough to make it to the Olympics, and now I'm going to have to cross this other obstacle."

The International Olympic Committee's 2009 decision to cram female boxers from 10 traditional weight classes into just three divisions _ at 112, 132 and 165 pounds _ raised safety concerns and altered the entire structure of a sport that's still chasing worldwide acceptance. Many fighters spent the past two years struggling to put on or take off a few pounds to fit the IOC's guidelines, while others didn't even try.

"For some of them, that's what ended their Olympic dream," said Christy Halbert, a veteran coach and the Chair of USA Boxing's Women's Task Force.

Six pounds might not seem like much, even on Esparza's compact frame, but it's a problem in such a finely calibrated sport. From Floyd Mayweather Jr. to the newest club fighter, boxers of all sizes obsess over the strength and mobility contained in every pound lost or gained.

"Some boxers that we've seen worldwide just figured they couldn't do it. They were too small to go up, so they didn't really feel like they could be competitive, or they were too big to try to make that 165," Halbert said.

Just 36 fighters _ 12 in each of the three weight classes _ will appear in the first Olympic women's boxing tournament this summer; trials began this week at the Northern Quest resort-casino outside Spokane. When the IOC created its weight classes for women, the governing body cited its need to limit the total boxers at the Games to 286; one of the 11 normal weight classes for the 250 male boxers was also cut.

After her initial dismay, Esparza _ a six-time national champion _ began the two-year process of meticulously building up her body to another weight class, approaching it as a challenge both athletic and intellectual.

"I take this extremely serious, and I had to make sure I did everything the right way," Esparza said. "There was no way that I could handle the strength of the other girls if I just jumped up, so I decided to take a whole year to build my fighting."

For the first year, the 5-foot-3 Esparza stayed at 106 pounds, honing her technique instead of her body. When she started to gain weight, she stayed out of the ring for a full year. She changed her diet, nearly eliminating carbohydrates and downing protein shakes and supplements.

For a year, she ate four or five times every day, yet never got above 109 pounds. The last three pounds came on in the past few months: "Now, I'm solid," she said.

Other fighters faced even bigger challenges. Christina Cruz, a longtime 119-pounder from Manhattan, had to go in the opposite direction to meet Esparza in the middle _ and she did it more quickly, honing her diet and focusing on speed over pure strength.

As a taller girl in a lighter class, she's finding ways to use the new weight to her advantage.

"It was a little different, and it took a while to get comfortable, but I like it now," Cruz said. "The girls at the lower weight are a lot quicker than what I was fighting. The target is also smaller.

"I'm one of the tall girls now," said the 5-foot-5 Cruz, "but a lot of times it works to my advantage. I can use my reach on them, and I think my speed has increased."

Even fighters who succeeded in making weight had to bid goodbye to their pre-eminence in another weight class. Andrecia Wasson won the welterweight world title in 2010 and was the world's top-ranked 152-pounder in November 2010. She had to move up to 165 pounds in just over a year to take her Olympic shot, entering the U.S. team trials as an unseeded fighter.

Other American fighters couldn't manage it.

Cashmere Jackson was the 2010 national champion at 141 pounds, winning a bronze medal at the world championships. The Cleveland fighter moved down to 132 pounds last year to fit into the Olympic class, but didn't make it to the trials.

Denise Rico was the world's No. 7 fighter at 178 pounds, but the Olympics have no place for her.

The shrinking weight classes have set up furious competition across the world for the debut Olympic spots. Just one fighter in each class at the U.S. team trials will advance to the world championships in China in May, where they must finish among the top eight to qualify for the Olympics.

Even before the classes merged, Esparza's 106-pound class was already the most popular "for all the small girls in the world," she says.

"Now everybody from two or three weight classes is at 112," she said. "But I feel good about it. I never had strength as a boxer before I gained the weight, and now I have it. I have everything now."