Scan at 38 weeks ?

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Feeling worried my last midwife appointment at 36 weeks I got measured and was at 30 percentile so got sent for a scan because my bump was usually at 50 when I went for the scan they just sent me out the room and came back out and said everything was fine woke up this morning to a letter for another scan will be at about 38 weeks I think ? Not sure why they would send me for another does anyone know ?