Honestly mama, usually ang first ultrasound (kung first trimester yan) ang most accurate for dating kasi mas consistent pa ang baby development pa, pero kung both pelvic ultrasounds lang yan in later pregnancy, the difference between them could just be normal baby growth variation, honestly, mas important na tingnan ng doctor mo kung which due date makes more sense based sa baby's size at development sa both scans, kaya I'd really suggest bring both ultrasound reports sa OB mo para makita niya which one is more reliable given your specific situation. A few of us moms in our mommy group had similar situations and ang OB namin ang nag-decide based sa overall picture, hindi lang one scan lang, so definitely worth the chat with your doctor po!