The way I understand it, the pit can also force harder contractions. Harder contractions can make for a more forced descent, which can leave the pelvis with less time to expand. A less expanded pelvis give less room for a baby to maneuver, and less room for a care provider to do the necessary maneuvers to reduce a shoulder dystocia. Also, pit has a known risk of fetal distress, and the dead last thing you want going into a situation where shoulder dystocia is a greater risk is to have that baby's oxygen already somewhat compromised.
That is why I personally plan home births but would evaluate measurements on ultrasound for a baby with weight suspected to be 10 lbs or more to see how head circumference and abdominal circumference match up. And if the head circumference was significantly smaller than abdominal circumference, I would go for the elective c-section. If the baby size is that big a risk, my personal feeling is that I would feel safer going for the caesarian. A 3 minute dystocia where your baby is born essentially dead really does color your view on things.
I'm going to assume you mean that normal weights aren't a risk for shoulder dystocia, not that true 10 and 11 lb weights are not a viable medical reason for an elective c-section. It's not a clear cut case like a placenta previa, but one where people's individual comfort levels, histories and prudential judgment really do come into play.
Shoulder dystocia and macrosomia are complicated, multifocal issues that really can't be reduced to "OH NOES!! THE APOCALYPTIC LARGE INFANT OF DOOM!!!" or "pfft.. that's fine! Your body wouldn't grow a baby you couldn't give birth to, and babies ALWAYS come when they're ready!" I don't trust ultrasound measurements of fetal weight, because I found a Henci Goer post citing a study where 60% babies referred for ultrasound for suspected macrosomia and identified by ultrasound as macrosomic were found at birth to be under the 4000 gram cut off. That's obscenely inaccurate. But I would trust my own feelings coupled with my midwife's judgment that by palpation an infant was around the 10 lb mark, and that's where *for me* concern about size starts to get really understandable.
To the OP, I hope that everything goes as smoothly as possible and happy babymooning!