Conception

How do you guys determine time of conception if you don’t know when your last period was? My doctor changed my due date today and my estimated time of conception doesn’t match up when I had a positive ovulation test so I’m wondering if baby will come late? I had a positive ovulation test on April 6th. My new due date is December 26th. My ultrasound scan today measured me at 8 weeks 4 days meaning I conceived around the 4th. Is it possible to conceive before you ovulate or ovulate again after you’ve conceived? I didn’t have a positive pregnancy test until April 16th.