Week 19 - according to babycenter.com

Your baby weighs about 8 1/2 ounces, and measures 6 inches, head to bottom — about the size of a large heirloom tomato. The hair on her scalp is sprouting. This is a crucial time for sensory development: Your baby's brain is designating specialized areas for smell, taste, hearing, vision, and touch. Some research suggests that she may be able to hear your voice now, so don't be shy about reading aloud, taking to her, or singing a happy tune if the mood strikes you.