To be fair, different health authorities do them at different times. And you're right, if there is anything unusual about the pregnancy or the circumstances, they will do them whenever they are deemed to be needed.
Also, if she didn't know until 20 weeks, there is likely to be a bit of guesswork involved in the EDD and how far the pregnancy has actually gone. When they aren't 100% sure they measure the foetus on the scan, and calculate (going by the average size of a foetus) how far along it is based on the size. This has its obvious drawbacks for instance if, like me, you grow large babies. I had my 12 week scan with Baby Yudster and I KNEW it was 12 weeks (I knew when I had conceived) and they measured him and told me I was actually 16 weeks, and re-calculated my EDD accordingly. I ignored this because I didn't think it would make much difference, except that I delivered early. However - the hospital insisted that their date was correct, and that I was therefore only 2 weeks early which is nothing to worry about. I however knew that I was actually 6 weeks early, which very possibly MIGHT be something to worry about. When he was born, he was big - so again, they assumed full term, fine - but he had minor breathing difficulties. Because of his size this was ignored and not treated which I think contributed to his cardio-pulmonary problems as a baby and toddler.
If he had gone to term though he'd have been about 12lb I reckon, so on the whole I'm glad he didn't!
Charlalottie on Twitter wrote:Just remembered that I played pool with a satanist last night. Really should go out on a Friday more often.