Due date if pregnant right after stop pill

Anna • Mummy to an angel 👣 survivor of early pregnancy loss 🌼 Hopefully second time is the charm 👣

My partner and I got pregnant straight after I stopped taking the pill (stopped 4 days into a new pack) before I got a real period (had withdrawal bleed). I'm wondering if anyone knows if they still work out how far along and the baby's due date from the first day of my last period since it was a withdrawal bleed not a proper period?