I’m not sure you have a case if the percentage of women on maternity leave in the fired group is roughly the same as in the non-fired group.
If it isn’t illegal to fire people taking maternity leave specifically, which I don’t think it is in the US, you’re out of luck. The only illegal thing is firing people because of maternity leave. Since there was a mass layoff, it can easily be argued that the maternity leave was not the reason.
The US needs better labor laws, and thus unions. An individual can’t do anything against it.