No matter how many ultrasounds you have during your pregnancy it is the first one that will help determine your due date. If the ultrasound due date is within a week or so of your calculated due date by your last menstrual period then we do not change your expected delivery date. The later you get in your pregnancy the more inaccurate your calculated due date will be. Some babies are born six pounds while others ten pounds but early in pregnancy all babies are about the same size. So once you have your first ultrasound whatever due date we determine is your permanent due date.