EXACTLY 15 months ago, President Bush vetoed a bill that would have allowed employees to take up to 12 weeks of unpaid leave to care for new children. Since then some 5 million babies have been born in the United States, nearly half of them to mothers who work outside the home. But without federal legislation guaranteeing a job-protected leave, many of these new mothers have had to return to work within six or eight weeks of giving birth - or risk losing their jobs.Now Congress is trying again with an amended family leave bill. The measure exempts companies with fewer than 50 employees. It also excludes the highest-paid 10 percent of a company's workers. In all, 95 percent of businesses - and half the work force - would be exempt. Congressional approval of the measure appears almost certain - but so does another presidential veto. Business leaders oppose mandated leaves, arguing that the policies represent unwarranted government intrusion. They also maintain that the cost of unpaid leave would be high - $330 million a year, according to the General Accounting Office. But that amounts to just $7 a year for each covered employee. Even those expenditures could produce benefits. A study by the Family and Work Institute, a research and advocacy group in New York, found that businesses with family policies actually cut costs in the long run because they increased the retention and loyalty of trained employees. Although 13 states have family-leave laws, millions of workers elsewhere are unprotected. Pediatricians and experts on the family emphasize the importance of the first three months of a baby's life. This is a time, they say, for parents to get acquainted with their baby, establish schedules, and cement family bonds. Ideally, employers would voluntarily offer job-protected leave to new parents - fathers as well as mothers - making federal legislation unnecessary. But in the absence of widespread corporate policies, passage of the family leave bill would acknowledge that getting babies off to a strong start is important for families - and ultimately good for business.