Ay, congrats sa first baby mo! So if your last period was April 19 and your due date is December, that means you got pregnant around early May, usually doctors count conception happening around 2 weeks after your last period, kaya roughly May 3-5 ang actual date ng buo. Pero honestly, first trimester ultrasound ang most accurate way to pinpoint exact dates, kaya i-compare mo na lang with whatever the OB told you during your first scan!