In recent weeks, labor unions and women's groups have received complaints from women across the country who were told they would not be hired because they were pregnant and others who were warned they would loose their job in case of pregnancy. Union leaders said many employers had already signaled that they want to replace young women with men.

''We cannot prove there is discrimination because it is disguised,'' said Jorge Barros, who heads an employment bureau in the Ministry of Labor. ''But there is a clear change. Companies are starting to ask for both sexes instead of only asking for women. When it comes hiring, then they choose men.'' While no statistics are available yet, Mr. Barros said, the change is notable for jobs like low-level office work, hotel staff and cleaning services.

Similar complaints have come from associations representing shopworkers, secretaries and bank employees who said that a number of employers have made it clear that their new though unwritten policy is that they will no longer hire women of childbearing age.

''Store owners argue that even if women do a better job, they have to hire and train a substitute for every pregnant woman,'' said Alceu Cabral, head of the Federation of Shopworkers Unions, which says almost 40 percent of its 1.6 million members are women. To four months' leave the store owners add another month's legal holiday, said Mr. Cabral, ''and so they lose interest in a young female employee.'' Men and Women Employers

The new prejudice against pregnancy has appeared among both men and women employers. ''I just cannot afford so much turnover in my workshop,'' said a woman who owns a garment factory and asked not to be named. She said she employed one man as a bookkeeper and 33 young seamstresses. ''I will gradually replace them all by older women and men,'' she said.