At 3 to 4 months, she'll push up onto her hands and lift her torso from the floor.
As your child rounds the 2- to 3-month mark, she'll begin to be able to lift her head when she's on her belly—and this represents a real opportunity for her to interact with her world. "It's a chance to start looking around, to do something different from what she's been doing," says Karen Petty, Ph.D., a professor of early child development and education at Texas Woman's University in Denton, Texas, and the author of Developmental Milestones of Young Children (Redleaf Press).
From there, your baby will make incremental advances. At 3 to 4 months, she'll be able to move her head from side to side, and at 4 to 6 months, she'll start to master "push-ups": She'll push up onto her hands and lift her torso from the floor. "It's all transitional," Petty says. "Every developmental act plays into the next." The end goal? Walking. "Somehow, innately, babies know that this is what they have to do, in increments, to get upright," she adds.
To help her get there, be sure to give your baby plenty of short stretches of tummy time—and don't fall into the trap of keeping her in your arms (or a swing, bouncy seat, etc.) too much. "You have to commit to several periods a day when your child isn't in a restraining device," Petty says. Also keep in mind that building the muscles necessary to master these milestones takes a lot of work—consider how heavy her head is compared to the rest of her body, and the work required to simply lift it—and that you need to keep an eye out to make sure sheisn't getting stressed or overly tired. "If she is, pick her up and soothe her, then put her down again later," Petty advises.
Now a word of perspective: Some babies may reach their milestones at a slightly earlier or later age than their peers. That said, if your baby isn't showing dramatic head movements by 4 months, call your pediatrician. Chances are everything's fine, but a little reassurance can't hurt.