Why do they go off lmp? Is it possible to conceive after

Audrey👬🌈🤰 • Married mommy of two handsome boys 👬 5 &7 miscarried July 2018 🌈 and now pregnant due November 2019 God willing!🙏🏽

🤔 I wonder if my due date will change going off of lmp april 2 they had said due date jan 14 but when i went for the 12 week nuchal ultrasound they said it was too early baby was measuring at 10 weeks so im scheduled for an ultrasound july 16th now so its scaring me is baby growing slow?? Or am i just not as far along as I thought...