Are they experienced ewes or first-timers? Did you use a Shetland tup or something larger? Have you scanned, do you know how many lambs they're carrying?
The best guide is to keep a close eye on body condition and if any ewes are carrying less condition than about CS2.5, give them a bit of feed with the aim of not letting them lose more.
All ewes need a lot of glucose in the final 6-8 weeks of gestation, which may mean they'll need a bit of hard feed or molasses from early Feb, depending on where you are and when you start to get grass growth.
We don't generally feed our Shetland and Shetland x ewes but they all go to a Shetland tup to lamb from mid-April, and we're in North Cornwall where we often have grass growing most of the winter. (But sometimes it doesn't, or is too wet to be useful, so then we might feed a little in March.)