Genetic (fraternal) twins are passed down through the maternal line b/c the genes need to affect you and how you ovulate- your husband's genetics don't affect your chances, but your genetics do! So that is definitely something to consider. Though I don't think it's common to miss twins in a 7/8 week u/s it certainly can happen. There is a woman in my DDC that had a 11 week u/s where they missed identical twins that were found a week later at the nuchal fold u/s! A doppler is even less likely to pick up two distinct heart beats. You mentioned you had a 7/8 week u/s? The date they gave you in that u/s would be quite accurate- within days. A dating scan at this point would have a much higher margin of error- more like 1+ weeks. So if you did get a date from that early sono you would not be farther along than that date.
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If you feel the need to know if you've got twins in there prior to the 20 week anatomy scan (and I can see why you would!), you would need to push the issue with your care provider and ask for another u/s. But if your care provider doesn't suspect anything is out of the ordinary then the likelihood of twins is pretty low b/c of the variation in fundal height prior to 20 weeks.
