An ultrasound scan is the only way to confirm that you're pregnant with twins. The earliest you're likely to find out is between 10 weeks and 13 weeks plus six days, when you have your dating scan (NCCWCH 2011, McAslan Fraser 2012). Ultrasound is almost foolproof at diagnosing twins. However, if you're carrying more than two, it gets harder to tell how many babies you're expecting.