Can a healthcare provider diagnose twins without an ultrasound?
Not conclusively. One of your health care provider's first clues that you might be pregnant with twins would be if you're measuring larger than expected for the gestational age of your developing baby (or babies!). Generally, the size in twins may be 1 1/2 to 2 times greater than expected in the first and second trimesters. Any fundal height above the navel at or before 20 weeks is worth investigating further.
Finding two heartbeats may seem like the solution, but it's difficult for a provider to pinpoint two separate heartbeats. Babies move around—so whether you're hearing two heartbeats or just the same heartbeat in another place is unclear.
Normally, your provider will schedule a regular ultrasound appointment for you between 16 and 20 weeks gestation. An ultrasound—short of delivering 2 babies—is the only way to conclusively determine that you're expecting twins.
Keep in mind, it's important for your provider to determine you're pregnant with twins as soon as possible: twins are considered a high risk pregnancy so your provider will take extra precautions to make sure your babies are healthy.