What Causes The Difference In Gestational And Foetal Age?

my first period date is 19 aug 2014....according to this date im 24 weeks pregnant...but a few days before my doctor took my ultra sound n it was showing pregnancy of 22 weeks...which then my doctor suggested me that my baby is 2 weeks weaker n need some care....this is my 3rd pregnancy ? can it be safe pregnancy....i hav two daughters with normal delivery....