bakar on "One twin measuring behind"

I am 18w4d today and had my anatomy scan where baby A looked good, but baby B is measuring 3-4 weeks behind (was normal at last scan 7 weeks ago). They stopped the exam after only looking at the brain and heart - I thought it was because the tech was having a hard time getting baby B on the screen (took an hour of me constantly turning and her pressing the probe HARD into me) but the peri said no it was because the baby was too small.
I was just wondering if anyone had this much difference in size this early and what the outcome was? The peri is insistent on an amino (which I don't want to do) saying it is chromosomal at this age and that if I don't do the amino they will have to take both babies at 24 weeks and therefore compromise baby A for baby B that may not live anywhere if it has trimosy 13 or 18.