Legal implications of paying a bonus to females returning from maternity leave as a retention tool

What if any legal implications are there in having a policy of paying a bonus to females returning from maternity leave as a retention tool to increase the number of female employees in a male-dominated industry?

So I suppose the background here is there aren't that many females. They're losing them as they go off and have children. They're trying to keep them there, and so they're saying, "Look, we'll keep the bonus payments going." And if you come back, it's almost like a golden handshake for coming back. Men don't get it, presumably because they're not off on maternity leave, and they haven't had the absence. But there may be an issue here around, well, what if they took paternity leave?