baby measuring ahead

I had an ultrasound today at my dr appointment. According to my lmp I'm going to be 31 weeks tomorrow, but my baby is measuring 34 weeks already & weighs an estimated 4 pounds. Dr. says he's already head down and aligned for delivery. When I had an ultrasound at 20 weeks he was measuring a week ahead. Has this ever been the case for anyone's previous pregnancies? If so what was the outcome? I'm a ftm, so I'm confused lol does this mean he may come early? Could I have mistaken spotting  for my lmp? I don't remember what my flow was like for my lmp so that adds to my confusion lol 😂 thanks for any help guys😊❤️