How tall is she? build? What has she been getting up til now?
Generally, a good mare/foal (or growth) feed along with good hay and/or pasture is recommended for lactating mares, but you don't want to switch her over too quickly from what she is used to.
And, you can post a "baby announcement" without pictures, even if you came to ask for feeding recommendations.
Please let us know what she's been eating up to now, how old baby is, and a picture of her from the side right down at her level will give us an idea of her body type and how she's carrying her weight. Then, we could give some better "educated" help to you........plus, we LOVE to see pictures of babies and mommas!!