If you have a heat-lamp, you can place them under it to keep them from getting chills, but you should make sure the heat lamp isn't too hot. As a general rule, it's bad to blow-dry a guinea pig, as the heat from the dryer can burn their skin. Take care to make sure there are absolutely no drafts where they are drying to prevent chills. Try to limit the number of baths you give during the winter, as most homes tend to be cooler and more drafty during this time of year. Also, don't use these shampoos on pregnant sows unless under the direction of your veterinarian.
Some owners recommend that no baths be given at all to pregnant or nursing sows.
Again, consult your veterinarian.