When we first count the dates, we base it on our last menstrual period (LMP). That’s how you got 12 weeks. But the scan done is called the dating scan, which uses the baby’s measurements, and also compensates based on how long your menstrual cycle is each month, so the actual gestational age of the baby can be quite different from the dates counted based on LMP. The dating scan is usually more accurate, so take it as 9 weeks:) and congrats!