Last winter, Ms. Hairston told the human-resources department for Planned Parenthood’s clinic in White Plains, N.Y., that her high blood pressure was threatening her pregnancy. She sent the department multiple notes from her nurse recommending that she take frequent breaks.

Managers ignored the notes. They rarely gave her time to rest or to take a lunch break, Ms. Hairston said.

“I had to hold back tears talking to pregnant women, telling them to take care of their pregnancies when I couldn’t take care of mine,” she said. “It made me jealous.”

Discrimination against pregnant women and new mothers remains widespread in the American workplace. It is so pervasive that even organizations that define themselves as champions of women are struggling with the problem.