Kicking is one of those behaviors you should NEVER discipline "nicely" for, nor should you EVER make excuses for it like "she is preggo so she is grumpy".
This mare needs to think the world as she knows it will end if she EVER tries that again. That has to be accomplished quickly and without leaving any question in her mind. How you do it depends upon what you have available to you, where you are in relationship to the horse, etc. Ideally, yes, grab one end of the lead about 3'off her head, and take the other end in your other hand with at least 3' hanging and go right for the side of her butt so she is forced to have her head toward you, hind away and go around you quickly. I just swing it in a rapid circle so that it gets her butt, and make lots of loud obnoxious sounds, like "AAARRRRRGGGGHHHHH" So that eventually she will associate that sound with the lead smacking her, and you can do that when she starts and nip it in the bud.
Like I said tho, it depends upon where you are, and what you have avaiulable. I have had horses cowkick at me, to which I typically kick them back, tight in the belly, again, yelling. I do not make a habit of hitting or kicking my horses, however, I am a firm believer in force meeting like force. Just like would happen in the herd. I also know that nothing I could inflict could even compare to the wrath of an alpha mare who is pissed off. She would definitely not put up with that crap either.
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