To your second question: this is a common scare tactic used on women. Not only is an ultra sound not acurate in determining the size of a baby in utero, but the pelvis is made of bones that are not attached all the way around. The pelvis can stretch and give lots of room if you are giving birth in a position that allows for this to happen (not on your back).
This is an article I loved reading about the pelvis
http://www.midwiferytoday.com/articles/pelvis.aspPlease read this study about ultra sounds
http://www.lamaze.org/NormalBirthForum/tabid/363/forumid/11/postid/30413/view/topic/Default.aspxI know many women who have given birth naturally to 10lbs babies or more. Some of them were just a little over 5 feet tall. Do not let this doctor scare you into a convenient for him c-section. What is his rate by the way, do you know? May be good to ask. If it is high, and esp. with this recommendation, you may want to get really educated about common practices that are not evidence based and what alternatives you have. He will become relentless. A good doula may be a good thing for you to have at your birth.
maria.