Question about ultrasounds

Hello ladies,

I am currently 35+3 & my question is how do the doctors know if there isn't alot of fluid around the baby? if the umbilical cord has the correct veins? If everything looks good inside and if they baby is growing correctly... idk if its a dumb question I am just curious also how would they know if my baby is measuring ahead or behind....I had my last ultrasound at 17 weeks and that's all for ultrasounds...