Certainly not true.
There are serious conditions that cannot possibly show up before 20 weeks. That's why most other countries offer monthly scans.
I think the UK should offer monthly scans as a matter of course.
My daughter measured correctly at her 28 wk midwife check and was falsely reassured.
At 30 wks she happened to pay for a 3D scan and was sent straight to her local hospital.
There she was told that the baby had duodenal atresia (which cannot be picked up until 28 wks) and was small for dates, she also had polyhydramnios. With these findings the baby was given a 40% chance of Downs Syndrome. She was eventually sent home with lots of leaflets and DVD information on this syndrome. She was devastated.
She had an amniocentesis the same day and had to wait an agonising 16 days to have it confirmed that the baby did not have chromosomal abnormalities, although she had, by that time, come to terms with having a Downs baby.
The amniocentesis collapsed the membranes around the baby and she struggled with contractions for another 4 weeks before the baby was born. They were trying to stop the contractions and get her to 36 wks as the baby was very small due to the atresia.
Luckily, because she 'accidentally' found out about the atresia, a surgical team was arranged well in advance and her baby had surgery within 12 hrs of birth. There followed a long 6 wks in NICU (was told to expect two weeks in NICU) as there were serious complications as a result of the surgery.
The surgeon told her that it's known that some babies are born with this condition without prior knowledge and it can lead to tragic consequences as the babies are fed, inhale the contents of their stomachs into their lungs (due to complete blockage) which has led to some deaths.
So we thank god that she decided to have a 'frivolous' private 3D scan.
So there is a very good reason to be scanned monthly during pregnancy.