Slow embryo growth

Hi, as a curious mother-to-be, what does slow embryo growth mean? My first check up was on 21st Oct and obgyn said it has been 4-5 weeks. But then on 11 Nov obgyn is uncertain of the foetus as it cant be seen but only the yolk and sac can be seen. I have to wait for another 3 weeks to come back for check up and see how it goes. Is this normal? Kind of worried :(