Don't worry! The Dr.'s do NOT know exactly how big the baby is!!!
I went in a month ahead for an ultrasound checkup, and the nurse went ballistic (3rd child of mine) thinking I was due any day, and when she found out I had a month and a half to go, was putting me on all kinds of tests, etc. for 2 hours....she turned the screen from me, I heard the baby had to be terribly cramped, etc. etc....so they did stress tests, etc.to which they finally sent me home.
I found out that measuring is "approx.", and "the measurments" are only approx. estimates, etc.
I delivered at normal time.....my baby girl was 6 lb. 12 oz. nice and easy size. Nothing huge like they said I would be! (They told me at least 10 lbs.!)I also only gained 20-25 lbs. with all my kids....and it was wonderful! I'm sure you're "all baby". Just eat healthy veggies and fruits, and you'll be Great!...and so will your little one! Amnio's are very dangerous, as they are invading. (Sticking a needle into the amnio-sac can introduce germs, etc....) Not needed. Also, if a baby is born prematurely, when their lungs are exposed to air, they develope 2-3 times faster.But if there isn't a need, then it's best for them to finish on their own! ~ My sister has had 8 healthy babies, and one was a month and a half preemie. (Big 25 year old now....very handsome and full of muscles!) She learned she's the responsible one, and the Dr.'s work for you. Do your research on the web, ask experienced parents, go with your gut, you're the mom, and if something doesn't seem quite right, examine/research it for yourself , and do what you think is best.
Remember, the Dr.'s are working for you, you are paying them. You are ultimately the one responsible for your health, as well as your baby's health. If you feel great, the baby is moving well...I'm sure you are both healthy, and there isn't anything to be alarmed of! Enjoy your time together before she's here!...life will be forever changed. Wonderful in a different way. :)