BOURN, England (AP) -- When Louise Brown cut a big, frosted cake at a huge lawn party Saturday, she was celebrating far more than her own 25th birthday.

Brown, the world's first test-tube baby, marked the anniversary of in vitro fertilization, a technique that revolutionized treatment for the infertile and has brought about the conception of more than 1 million children.

Hundreds of IVF babies and their families gathered to celebrate the medical milestone at Bourn Hall Clinic outside Cambridge, founded in 1980 by Dr. Robert Edwards and the late Dr. Patrick Steptoe.

"Louise's birth signified so much," Edwards said. "For the first time, science and medicine had entered human conception in the most decisive manner."

"We did something. It took some doing. We had to fight a lot of opposition, but we had concepts that we thought would work and they worked," he added.

Edwards was flanked at a news conference by Brown and Alastair Macdonald, the world's second IVF baby.

"Without Bob, we wouldn't be here," said Macdonald, 24. "It's wonderful to see all the children that have come through IVF. It's amazing to see it's come so far in the last 25 years."

Brown, who was born on July 25, 1978, and is a postal worker, said she didn't consider herself special. "I just get on with my life," she said. "Just normal -- I just plod along."

Asked how it felt to be sitting next to Edwards, she giggled and said: "Weird. I'm shaking."

Edwards' and Steptoe's pioneering technique -- combining sperm and egg outside a woman's body and then implanting the resulting embryo in her uterus -- utterly changed the options available to couples with trouble conceiving.

"If I may be very selfish, I'm delighted that the same techniques are working all over the world," Edwards said. "They have improved because science has advanced."

The probability of an infertile couple taking home a baby after a cycle of IVF today is 1 in 5, about the same chance that healthy couples have of conceiving naturally each menstrual cycle.

Nonetheless, the procedure can be difficult and costly, and many couples are still unable to become parents.

Steptoe and Edwards were working on their revolutionary technique in the northern English city of Oldham when John and Lesley Brown first visited them.

Lesley Brown said she had faith in the men from the start. Her younger daughter, Natalie, was the world's 40th IVF baby, Bourn Hall said.

"When you've been through a lot of treatment over the years ... then any chance you get that someone's going to help you, you take it with both hands," she said. "I just completely always trusted them. I just knew they would help me, which they did."

Edwards noted that IVF still has frustrating limitations because only 20 percent of human embryos -- whether they're created naturally or outside the body -- implant in the uterus. He said he hoped to see fertility technologies improve further.

"We've got to keep on doing research on embryos ... we must understand early human embryo growth to improve it," he said. "The world is moving so fast in science."

Louise Brown said her parents told her about her unusual conception in simple terms when she was 5, showing her a video of her birth.

Macdonald, the first boy born through IVF, explained that he figured out his origins at age 9 while watching news coverage of Steptoe's death. The doctor had been a grandfatherly figure in his life, but no one had explained his role, he said.

Macdonald told reporters he ran upstairs after putting together the clues and shouted, "Mom, I know why I'm special!"

Copyright 2003 The Associated Press. All rights reserved.