When Anne Halsall, 34, brought her first son home from the hospital in 2012, she eagerly followed the best advice about breast-feeding. Her son, however, kept losing weight — first a little, then a lot. “It was a dark time for me,” she said.

After getting conflicting advice from experts, Ms. Halsall, a Chicago native who was living in San Francisco with her fiancée, then did what many frustrated new mothers do these days. She turned to Google.

“That’s when I realized I was a second-class citizen in the eyes of the internet,” she said. “I tried to download an app for breast-feeding, and they were all clearly made by men, and they were all horrible.”

So Ms. Halsall, an engineer, wrote her own, called Baby’s Day. “I was a frustrated mom who built an app for moms,” she said. “You can’t get more millennial than that!”