Question ???

Baby is measuring a week behind I was supposed to be 27 weeks at my last appointment check up but the belly height measured 26 and also the ultrasound said 26w 3days but at 27 weeks baby supposed to weigh 2.2 and he does…so how come I’m measuring a week behind ?

Also one time I was measuring a week a head so I’m just confused but they say they always goes with the first ultrasound which is more accurate

First ultrasound due date-oct 5

Second ultrasound due date-sep 30

3 ultrasound due date-oct 5

4 ultrasound now says -oct 10