Twins but only one heart beat, one looks smaller then the other, he thinks it's a mass twin, what is that? and is it possible he can be wrong?

Please have your OB: Explain everything to u so u understand it, but sometimes u can have twins and 1 is not viable and it usually gets resorbed so you end up with one healthy baby, obviously I am sure you OB is going to do a repeat scan in 1-2 weeks to check again.

Not common: Language. Your doc needs to explain what is rant better to you. It is not uncommon for on twin to be smaller than the other, but if a lot can be a problem. Often the position of a twin makes it hard to hear a heartbeat but if they see it move it is alive. Maybe you need to see a high risk obstetrician . Good luck!