No eggs for weeks and weeks

My two girls have not laid an egg for a very, very long time. It's winter. It's been super cold. Is there a point when I need to be concerned for their health? I don't rely on their eggs, so it isn't that. I'm worried about them.

This is their first winter (they hatched at the end of March). They are eating and drinking, although the amount of food they eat has gone down, but still putting a good dent in their food dish each day. They spend more time in their coop then in their run during the day, but I close them in their coop at night with a lot of bedding and insulation.

They were pretty regular in the summer/fall with an egg or two total between the two of them.