LAte scans can be really inaccurate though.
If she didnt know her dates they surely did a dating scan earlier on?

With DS1 I had the two "normal" scans at 12 and 19/20 weeks, both confirmed my EDD base on LMP to within 3 days.
I then had to have a scan at 36 weeks, which said my EDD was FOUR WEEKS earlier, and infact the next day.

But my dates werent wrong, I just had a 10lb_, very long bub.

Has she had monitoring done for placenta function?