If it was safe for you when you weren't pregnant, it's safe now. Just keep it pure, and to be extra couuiats ask for a sensitive or preggo mix. Sensitive skin mixes usually are just diluted juice and sugar, and won't have any essential oils other than lavender oil. If you are extra, extra couuiats, then ask for no essential oils at all, and rainwater that's been strained and boiled instead of lemon juice. Can't be tap or bottled water, you need some acidity, and rainwater is a gentle choice. When that bump gets big, your friend will be drooling to henna that baby belly. It's a time honored tradition, and preggos are rare enough that many artists never get to work on one.