Baby growing too slowly…

Hi mummies! Went for my second scan and doc mentioned that the growth of the baby is too slow and is not positive about it. This should be my 7th week. (During my prev scan on 5.5week, baby was 2.4mm. Today was only 3.9mm not able to find heartbeat through transv scan) - is this growth really too slow? Anyone has the same encounter with a good result in the end? #firsttimemom