heartbeats, maybe twins?

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I heard my babies heart beat for the first time yesterday(15 weeks, 5 days). It was the most amazing thing I have ever experienced. This is the first time I have ever heard any babies heartbeat and I have a question about it. The nurse put the machine very low on my left side and heard a strong heart beat, then lost it. Then she put it up much higher on my right side (within seconds) and there was a strong heartbeat. Both time I could hear a faint heartbeat in the background. My fathers mother was a twin and my man's mothers mother is a twin. What is the chances that the second heartbeat is a sign of more then one baby? I know that twins in the family skips a generation and now I am very excited and nervous. Do anyone know about twins? I don't have an ultrasound for about another month and I really want to know. Is there anything else I could look for, signs...