To determine your accurate week of pregnancy, it is best to rely on the guidance of your healthcare provider rather than just an app. Your doctor may use various methods such as measuring the size of the fetus or calculating based on your last menstrual period. It's common for there to be a slight discrepancy between different methods.
As for when to get an ultrasound, it depends on various factors including your medical history and any specific concerns. Generally, a dating ultrasound is recommended around 8-10 weeks gestation to confirm the due date and check for the heartbeat. Additional ultrasounds may be performed if there are any complications or if further monitoring is needed.
It's important to communicate with your doctor about any confusion or concerns you have regarding your pregnancy timeline and ultrasound schedule. They will be able to provide personalized advice based on your individual situation.