The base layer (or first layer) should be soft, natural and breathable. Like a second skin, a good base layer will not inhibit freedom of movement and will support the body's own way of regulating temperature. Base layers should also ‘wick’ (or draw) moisture away from the skin to maintain warmth and comfort day and night.
Babies and toddlers cannot regulate their own temperature as well as adults and heat is lost up to 4 times faster. This is especially true of newborn babies. Merino wool, because of its superior moisture management and antibacterial properties, is the best thing you can put next to their skin. Cotton is soft and breathable, but quickly becomes heavy, cold and clammy when wet and takes a long time to dry out so it is best saved for high summer wear. Merino wool is the ideal fabric for base layers as it stays warm even when wet and dries very quickly (perfect for catching dribble!).
base layers are designed especially with the British and Northern European climate in mind.
2. The mid layer – adds extra insulation on cold days and generally only needed if you are going out of doors in the winter. Adjust the thickness of the midlayer according to the temperature. In cold weather fleece is a great choice. Real wool fleece is the best for babies because it is breathable and continues the good work the base layer is doing drawing moisture away from the skin. Synthetic fleeces are less breathable but just as warm and as long as you use a merino base layer your child will still remain comfortable next to skin.
3. The outer layer is the weather protective layer should be like a shell to keep inner layers safe from the elements. For very cold conditions a padded outer shell or a double mid-layer will be needed for extra insulation.
The Superlove Guide to Layering