Does an empty sac ever turn out to be viable?

I had an ultrasound at 6 weeks today and had two sacs. One was a healthy little one with a strong heart beat and the other was measuring 4 weeks and appears to be empty.

I have had no cramping or bleeding. My ultrasound at 5 weeks showed only a little shadow of that smaller sac and the doctor thought it wasn't even a sac at all. Could this end up being a viable twin?

Any positive stories similar to this?

[AnnieBrooke]

If there was ever going to be a second embryo, it would be the same age and development (give or take 12 hours) as the first.  You don't ovulate once you are pregnant.  The second cell cluster probably never got any further than just to produce a sac, and then stopped developing.  I am sorry, it sounds like this is not what you want to hear.  You probably will not have any cramping or bleeding for just a sac, since you have a viable pregnancy.

[stardust0477]

This isnt always true. In my family we have history of ovulation from both ovaries in one month. IT IS possible to release eggs at different times... NOT common, but is possible. 3 of my cousins, as well as myself currently, all have twins, all 2-3 weeks different gestational age. The only time they are exactly the same is if Identical twins, in the case of fraternal, they can be a few days, with documented cases up to 3 weeks different gestational ages. Im currently pregnant, and still waiting to see if my second one will be viable because its dated 2-3 weeks behind the other, the sac is getting bigger, but no sign of fetal pole/ect yet. This is also how some mothers can give birth to babies with different fathers. Again only a few documented cases that are known 100%, but it does happen! Family history plays a huge role in this!