Question: how soon do you know you're having twins?
Answer: An ultrasound scan is the only way to confirm that you're pregnant with twins (McAslan Fraser nd, NICE 2011). The earliest you're likely to find out is between 10 weeks and 14 weeks, when you have your dating scan (McAslan Fraser nd, NHS 2019, NICE 2011). Ultrasound is almost foolproof at diagnosing twins.