First ultrasound at 4weeks and 4days

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i had some overy cramps then went to my dr and he did an ultrasound so he wrote in the report that my gestational sac is 15mm/14mm with large diamètre.and no yolk sac .and proceed by that the pregnancy age is 5weeks and 3 days.. what made me worry is the 15/14mm of the sac is there any similar conditions that ended in a healthy pregnancy..also my hcg are doubling every normaly