baby measuring large

I went for my 35 week appointment today and my baby is measuring at 37 weeks. So now I have to go back in on Monday for an additional ultrasound to determine if baby really is big or if it was positional. Now here's a strange question. I haven't had a decent bowel movement in a few days and I was having bowel cramps during the appointment. Has anyone ever heard of not having a bowel movement can cause ur fundal height to measure wrong? I have measured perfect up until today. However, my husband was a 10.5 lb baby. I do not have gestational diabetes, and I have had about 20lbs in weight gain over the last month. I have swelling and the dr thinks it's fluid weight, urine is fine and BP is fine. I'm just wondering if there's a possiblility my lack of bowel movements could be throwing off the measurements. I really want to have this baby natural if at all possible.