Is a dating ultrasound accurate showing 8 weeks if the fetus has iugr and woman has no known lmp? can the fetus actually be 20 weeks along and just look like 8 weeks due to iugr

Quite accurate dates: 20 weeks never looks like 8 weeks, just like a 20 year-old who was malnourished and neglected all his life will still not look anything like a normal 8 year-old boy. In a natural pregnancy, ultrasound dating that shows an 8 week fetus is an estimate that the pregnancy is 7-9 weeks. Avoid overthinking this, unless it is 100% impossible that there was a sperm meeting an egg 5-7 weeks ago.