US is not 100% accurate on guessing weight, gestational age, or whether everything is a-ok, and I do not know the rate of false positives. You do need to consider what you will do if something does show up wrong; think long and hard about that before having the ultrasound. What if you are told everything is fine only to deliver a baby with a life-threatening issue 3 hours from the hospital? What if you are told something is wrong only to deliver a healthy baby, but you've had to have a very interventionist pregnancy and birth? What are you more comfortable with? A US cannot offer certainty. In fact, if something shows up wrong it can affect the whole way your pregnancy and birth are handled, as well as causing a lot of anxiety. However, it can also offer reassurance. But both of these senses can be false.
It sounds to me like the real issue here is that you are worried something will go wrong and that you will be too far from the hospital. I think that is a very real concern. I don't think it has anything to do with a lack of faith in life, birth, or yourself; I think it is very natural for us to fear the worst and to prepare for it. I have been preparing for the worst but hoping and expecting for the best, minus the ultrasound. I think that is a good approach for anyone, ultrasound or not. My advice to you would be to do the same: prepare for the worst, expect the best. Do what makes you feel most comfortable. It sounds like you truly aren't 100% comfortable with birthing so far from the hospital...and while US may ease your mind if everything looks fine, I think it will be an artificial sense of reassurement. Your comfort with this decision needs to come from many sources, not just a US. I think a better thing to do would be to plan to give birth nearer to a hospital.
I have a great deal of faith in birth, but even so, one of the reasons I feel comfortable with my choice to UC is that there is a hospital ten minutes from my home. I also know from experience that the paramedics can be here in 5 minutes. A hospital with an awesome NICU is about 20 minutes off. (Tricare only covers a birth in the Army hospital, so even if I weren't UCing, I'd still have to transfer if there was a neonatal emergency.) I have the number of a midwife who is accepting of my UP/UC choice and who would be willing to see me if I had any concerns. I believe that this will turn out wonderfully and be best for my whole family...but I also know there is always that slim chance. Having access to prenatal care from such a wonderful woman as well as quick access to emergency care puts me even more at ease. I have gained a lot of knowledge since I began this journey, and my confidence has raised a little everyday. I have also taken precautions in case my plans do not go, well, to plan. I know that it's unlikely for any complications or defects to occur. I trust medical professionals' skills at handling these rare situations (just not normal births). All of this is what gives me the peace of mind I need to proceed.
I think one of the most important factors upon birth outcome is the stress level of the mother. If you feel like an ultrasound would put your mind at ease, then have one. But I think you also need to find other sources of reassurance..and as time goes by, you probably will. I would recommend waiting until a bit after 18 weeks if you do have an ultrasound, maybe 20 or 22 weeks. And again, have a good
plan in case of an emergency.
I am booking an ultrasound for later this week because I'm not sure of the exact location of the placenta. I can hear it best low in my uterus, but I also hear it up a little higher but more faintly. To make sure it is not in a position that will hinder a safe birth, I'm going to have an ultrasound. While we're there we're going to confirm how many babies there are (probably just one!), gender, and position...as well as see if we can get a weight estimate. We may as well if we're paying for it. I feel like that will put my mind at ease...and I trust this midwife, so if she says the placenta is somewhere that would make birth risky, I can take her advice whatever it may be--have a midwife there, be in the hospital, C-section.
In the end...it's up to you. Just think long and hard about it, because once you have the US and the results, there is no going back. Just like once you're 39 weeks along and in labor without having had any testing, there's really no going back from there either.
Good luck mama.
ETA - Woo, I sure wrote you a novel. Sorry!!