i have assisted during my last two hatches. I realized (after doing a lot more reading) that the Marans eggs I have don't lose enough moisture because of the thicker coating on the shells. As a result, the chicks grow too large and can't turn around to get in position for hatching. About half of my Marans chicks pip in the center of the egg instead of the fat end. I give them 18 hours to hatch, but if they don't make progress I end up slowly removing the end of the egg so they can get their head out and free themselves. I haven't killed a chick by assisting, but I have had some die in the shell because I left them too long and they didn't make it out. Of course the ones I assist always end up being cockerels, so I'm not sure what my rational is!