Is it possible to still be having twins after a 9 week ultrasound shows one baby and one heartbeat My stomache is really large to be just 3 months?

At 9 weeks a scan would have shown if you were having twins. Are you sure you are not just eating too much? I had twins and wasn't even showing at 3 months. Some women can hardly look pregnant right up until the 9th month, while others could sit a dinner plate on their tummy. It's doubtful you are having twins, but, you are obviously retaining a great deal of fluid (embryo fluid) and once you've had the baby it won't take that long to get back to somewhat of a normal state. Congratulations!