<prompt> Yolk sac is visible on USG first at4th week POGEarly 5th week POGLate 5th week POG6th week POG  <response>: 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 