In early pregnancy, the yolk sac functions as a source of nourishment for the developing baby before ultimately being absorbed by the baby as a part of the gut. The yolk sac is the first thing to become visible inside the gestational sac on a transvaginal ultrasound, appearing at an average of 5.5 to 6 weeks' gestational age.
When an ultrasound shows no yolk sac, the reason may be miscarriage but it could also be that the pregnancy is still early.
Because the yolk sac first becomes visible around 5.5 weeks of gestation, seeing no yolk sac on an ultrasound could simply mean that the pregnancy is not yet 5.5 weeks along -- and errors in remembering the last menstrual period or irregular cycles can affect calculations of how far along a pregnancy is at the time of an ultrasound. When doctors suspect incorrect dates as a possibility, they will recommend a second ultrasound. In a viable pregnancy, the yolk sac and possibly the fetal pole will always become visible on a followup ultrasound.
Seeing no yolk sac can also be a sign of miscarriage. When a followup ultrasound does not show continued development and the appearance of a yolk sac, a doctor will diagnose miscarriage. Research suggests that a gestational sac larger than 13 mm that contains no yolk sac means the pregnancy is not viable, so when the sac is larger than this and there is no yolk sac, the doctor can diagnose miscarriage based on a single ultrasound. Most often, a miscarriage that consists of no yolk sac would fall in the category of blighted ovum.
Unfortunately when you are faced with an ultrasound that shows no yolk sac, you will probably be told to wait for a followup ultrasound. The wait can be very difficult, naturally, but is necessary for an accurate diagnosis.
Concerns Regarding Early Fetal Development. American Pregnancy Association. Accessed 2 Jan 2009. http://www.americanpregnancy.org/pregnancycomplications/earlyfetaldevelopment.htm
Perriera L and MF Reeves, "Ultrasound criteria for diagnosis of early pregnancy failure and ectopic pregnancy." Semin Reprod Med Sep 2008. 26(5):373-82.