If I cut, I cut a small window about 24-48 hours after the first neonate has pipped, but only on some clutches. If I have possible hets that I'm trying to prove out, I'll cut windows just to see what I have. I haven't had any umbilical issues myself, but I could see how it could make the neonate emerge a little early causing problems like you've described.

This year I only cut a larger window after the neonate started to pip on it's own. I don't think there is any "good reason" to cut unless you have a two-headed specimen like yours.

-Rusty

Here's a pic of a clutch showing a few windows I cut in the eggs:

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