My Sister Just Told Us She is Pregnant

My sister just told our entire family she is pregnant but her HCG level was at 0 the doctor said. He said that she may just not produce HCG but they saw a baby with a heartbeat on the ultrasound. Don’t you have to have HCG to be pregnant? From my understanding it is the pregnancy hormone. My parents and other siblings are believing her. I’m just sorta confused because I didn’t think that was possible.