due date help please

L • 🇬🇧 Mummy to a 💗 4yo 2015 💗💗 2yo twins (One stillborn 🌻) 2018 💛 Ectopic March 2019 💗 Baby due Sept 2020

How do I work out my edd? I know I will be given one at my dating scan but even at my dating scan do they estimate of from the size of the baby? Or still go by your lmp?

Thank you