Low end of the year bonus and maternity leave. Legal question?

I am aware that a bonus is just that, a bonus, and that no one is "entitled" to it, per se. However, I received a bonus that was significantly lower than that of my peers (almost 5% lower). I have no performance issues, and just to be sure I confirmed it with my boss. He said that I'm doing a great job and that he is surprised himself that my bonus was so low. The only reason I could think of is that I took 6 weeks maternity leave (part of my benefits given by the company) and the standard postpartumum disability time. I have a meeting scheduled with HR about this, but prior to the meeting would like to know if it's legal for a company to "penalize" someone for using benefits that they are entitled to.