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What is Yolk sac is visible on USG first at and explain in detail?
yolk sac should be seen on transabdominal scanning when the mean sac diameter (MSD) is 20 mm or at a gestational age of 7 weeks and is usually seen endovaginally with an MSD of 8-10 mm or gestational age of 5.5 weeks.The gestational sac (GS) is the first sign of early pregnancy on ultrasound The threshold level identifies the earliest one can expect to see a sac (4 weeks, 3 days), and the discriminatory level identifies when one should always see the sac (5 weeks, 2 days).(Ref: Diagnostic Ultrasound by Carol M. Rumack, Stephanie R. Wilson, J. William Charboneau, and Deborah Levine,4thed, page 1079,1080)