I Had A Miscarriage Early This Morning Went To The

I had a miscarriage early this morning went to the ER due to bleeding and passing blood clots but all they seen was a sac but no fetus and after getting sent home my baby came out. how is it possible that out of 3 doctors they didn't see a baby through a vaginal ultrasoud or a regular ultrsound and sai i was an early ob and i was 11 weeks and 4 days pegnant? when I smybaby he was big enough to be seen and my calculation were corret in how far along i was and doctors were wrong with there calculations