From what I heard and read, you can get the amnio anytime. They often do them in the third trimester to check if the lungs are mature. And yes, it would be pre-term labor instead of miscarriage at this point. For what it's worth, I had my amnio when I was 26 weeks, they told me if I could wait about 4 weeks I should, so the baby would bake a bit longer **if** I was that worried about pre-term labor, but I was too stressed out to wait. The chances of something going wrong are so slim. One good thing about knowing now, besides a huge relief in most cases, is if there was something wrong, you can make arrangements to give birth where there are specialists etc. Also, (from researching) the femur and humerus lengths would have to be < 5 or at greatest >10th percentile to be a marker.
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