These are the measurements taken during a typical 20 week scan:
Cervical length (the full bladder is needed for this part)
Biparietal diameter (baby's head)
Amniotic fluid index
Other optional msmts, may/may not be taken depending on where you have your scan done:
Cerebellum (part of the brain)
Cisterna Magna (part of the brain)
Lateral ventricles (part of the brain)
So as you can see, there are multiple measurements taken of and within the head, which is probably what you are referring to. Even with a completely normal pregnancy there are just lots of measurements to be taken, and some are even repeated for accuracy.
During this ultrasound practitioners measure the size of the baby and check out all the major organs (it is called diagnostic ultrasound). They can determine the amount of amniotic fluid (maybe these are the shadows you are talking about) and evaluate th location of your placenta.
What other measurements they perform?
"Measurements of your baby's head circumference (HC) and diameter (biparietal diameter or BPD) are taken, along with the abdominal circumference (AC) and femur or thigh bone (FL), to date the pregnancy and to make sure your baby is growing normally".
You need to have full bladder as full bladder will push the uterus closer to the surface of your abdomen which allows for clearer pictures.
Its just routine to have a full bladder when having your 20 weeks ultrasound so they can make sure measurments add up.....It helps see the baby a lil better. EVerything is normal its nice when they tell you though that there not trying to be rude but will not be talking the first 10 min and taking ,measurements its kinda awkward haha, But its just routine hun. Most everyones appointmenst go that way. If there was anything wrong the doctor would have come in to talk to you! :) SO don't worry if she showed you the baby and went over everything and you found out the gender then your baby is in great health :)! Good luck and Congrats!