Cat/Cow is a common warm-up yogic exercise. It's escpecially good for women since it's the best way to release pent up stress in the spine. It also massages your organs in your reproductive system.
Here's the steps:

1. Come on your hands and knees in a table -like position
2. Inhale and lift up your head while you arch your spine down
3.Exhale and round your spine up and lower your head

Continue this movement for 1-3 minutes at a moderate pace. As your spine warms up you can speed up the movement. Use powerful breath for a better effect.