"Understanding how genes control brain cell migration, proliferation, differentiation, and death, and how radiation, drugs, toxins, infections, and other factors disrupt these processes will aid in preventing many congenital neurological disorders.... Recent studies have shown that the addition of folic acid to the diet of women of child-bearing age may significantly reduce the incidence of neural tube defects. Therefore, it is recommended that all women of child-bearing age consume 0.4 mg of folic acid daily."
http://www.ninds.nih.gov/disorders/cephalic_disorders/cephalic_disorders.htm
I would immediately follow the eating plan at www.blueribbonbaby.org to give this baby the optimal nourishment in the womb, especially for the remaining time.
The tape measure is wrong quite often because of the number of positions your baby can take in the womb at any given time. Your midwife is saying that the baby's head is not engaged, which means not ready for birth.
The scan does not accurately record estimated due date, weight or size of baby. Nor can it determine the amount of amniotic fluid, placenta or baby.
Stay calm and call your midwife again to clear up your concerns as they arise. Don't wait for the next appointment to talk to her.
Remember first-time mother's need up to 42 weeks, (dated from the first day of your last period - not ultrasound dating) and longer to prevent prematurity. Your baby, in particular, will need all the time in the womb you can give. Best wishes, G