Chelcie Gette’s mother called her early Tuesday morning to tell her she had gotten something in the mail. The two went on FaceTime and opened it together. “It was a cute little card congratulating me for my soon-to-be baby,” Ms. Gette, 27, said. There was one hiccup. “I knew I was not pregnant, so it was very confusing,” Ms. Gette said. “My mom thought this was my way of telling her I was pregnant!” It’s likely that thousands of women across the country in their mid-20s to early 30s received such a package this week: correspondence from a stranger congratulating them on their pregnancy, along with several gift cards for baby products. Many of the women who received it, though, weren’t pregnant. The card was purple and had a cartoon avocado with a heart over its pit. It read: “Holy guacamole! You’re going to avo baby!” Inside was what looked like a handwritten note, signed with a heart from “Jenny B.” It said: “Congratulations!!! I’m so excited for you! I hope you like these.” Included were a handful of coupons, five gift cards that amounted to $245 and a receipt proving their value. Lena Ghamrawi, a 27-year-old lawyer who lives in Silver Spring, Md., said she opened her own card...