London ? Twenty-five years after the birth of the first test-tube baby, success rates have doubled, but the quest for parenthood remains frustrating and expensive for most infertile couples.

The birth of Louise Brown on July 25, 1978, marked a revolution in fertility treatment. More than 1 million babies have been born through in vitro fertilization since Dr. Robert Edwards and Dr. Patrick Steptoe achieved their breakthrough in 1978 in Cambridge.

As Brown, a postal worker living in Bristol, England, celebrates her birthday today, fertility experts note the progress that’s been made.

“It’s always frustrating, but I think it has improved inordinately in the last 20 years,” said Dr. Francoise Shenkin, a fertility expert from the University of London.

The average probability of an infertile couple taking home a baby after a cycle of IVF today is 1 in 5. That’s about the same chance that healthy couples have of conceiving naturally each menstrual cycle, so fertility techniques have managed to restore the overall odds to normal, said Dr. Alison Murdoch, chairwoman of the British Fertility Society.

Even so, one out of every six couples struggle to become parents, and scientists have not yet been able to fix the biggest problem of all — a woman’s age.

“We can’t turn the clock back,” Murdoch said. “It’s a real tragedy when I get someone who comes along at 37 and they’ve been busy spending the last seven years of their lives getting the house right, getting the job sorted out and going on holidays and thinking ‘OK, we’ll have a family now,’ and they’ve left it too late.”

Although the specific cause of the infertility is a major factor, the mother’s age has become the most important determinant of IVF success as many of the other hurdles have been overcome with scientific progress, experts say.