Often, seeing no yolk sac (or a yolk sac that is smaller than normal or otherwise misshapen) at 6 weeks can be a sign of miscarriage. Unfortunately, you'll most likely have to wait until a follow-up ultrasound to be sure. 1 day ago · Ultrasound of a twin pregnancy Fraternal twins result from implantation of 2 genetically different embryos Yolk sacs and fetal poles (the early fetus) are seen in 2 completely separate sacs This pregnancy is at 6 weeks In such a case, there is a 40% to 50% risk that the healthy gestational sac and fetus will also abort The presence of gestational sac and yolk sac within.