Pippin weighed about one pound when she was 12 weeks old. She just turned 8 months old this week and now weighs 6+ pounds. So no, that method is not always accurate.
Actually, now that I think about it, it hasn't worked on any of my babies.
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He is your friend, your partner, your defender, your dog. You are his life, his love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion. -- Author Unknown