Re: How to do pelvic tilts
I recommend that you do this exercise on "all fours" (i.e. hands and knees position). That way your baby will act as a "weight plate" when you contract your abdominals in this position.
Start in "hands and knees" position with a "neutral spine" (i.e. your back is not sagging or arching). Inhale and let your belly relax; then exhale while "pulling your navel into your spine" while SLIGHTLY curling your tailbone forward. The abdominal contraction is actually pulling your pelvis into a "pelvic tilt" by shortening the distance between your ribs and your hips.
As your baby gets heavier, this exercise will get harder. Each day you will be lifting a little more weight than the day before? This is the ultimate example of "Progressive Resistance."
Good luck. Let me know if this helps.
Sheila