Problem with Egg! Weird color???

[RogueRubbDuck22]

Hey guys so this is the first time I’m letting my duck sit on a clutch of her own eggs and I’ve candled them the last few days and they’re growing. One egg that she’s had in her nest for about three weeks now has turned completely solid-ish (like it’s fully developed and should start hatching soon), however, I checked it tonight and I’ve noticed the egg has turned a different color than her other ones. I’ll attach a picture of it and of course my nosey call duck is in the picture but if any of you know what’s going on with the egg I would really appreciate the feedback and any suggestions on what to do with it!

The discolored egg I’m talking about it directly under and to the left of my ducks bill and the dark area on the top of the egg is actually the air sac.

 

[KathiQuacks]

Does it smell? Looks like a rotten egg imo. How long has she been sitting consistently?

 

[RogueRubbDuck22]

I would say that she has been sitting consistently for around or close to two weeks and I haven't smelled it yet.

 

[RogueRubbDuck22]

I got home from school, and I smelled it and it smelled gross, so it was definitely rotten. In the future, so I'm a bit more prepared, what would be ways to avoid getting more rotten eggs and what causes them so I can better prevent them from rotting in the future?

 

[Yardmom]

If eggs get cracked or dirty they are more likely to become "rotten" but any egg can go bad. Just check on the eggs every few days.