Expectant mothers can always use three things: a nanny, diapers and clothes. You may not be able to provide a nanny, but you can handle the diapers and clothes. But how do you buy clothes for someone who can't express their likes and dislikes? Good question. In this situation one thing that you can do is talk to the mom.
Don't be afraid to ask the expectant mother in your life about the types of baby clothes she is looking for. Ask her about the colors and styles that she would like to see her child in. If you think that asking that would be too forward, you don't have to ask. Just consider the mom's style and go from there.
For example, if mom is very trendy or flamboyant, she would probably appreciate trendy baby clothes in bright colors. If mom is passionate about the environment, she will definitely appreciate organic baby clothes. And if mom is a little more modern minimalist, she will probably want baby clothes that come in muted tones and don't have busy patterns.