Oo, totoo yan, late ovulation happens talaga, especially kung irregular ang cycle mo or kung may stress, diet changes, or hormonal stuff na nangyari around that time! Based sa transv ultrasound dating, that's usually pretty accurate, so if they're saying 4 weeks, possible talaga na naovulate ka later than expected even though your last period was October 20. Pero para sa peace of mind mo talaga, best na i-confirm ulit with your OB, especially kung may concerns ka about baby's development, they might want to do another ultrasound sa few weeks to recheck the dating and make sure lahat ay on track. Kailan mo na-confirm na ang pregnancy thru ultrasound, at ano sinabi ng doctor mo about the dating?