With my dd i had a scan at 6w1d and it dated her as 6w1d. By the time we got to 19wk scan she measured 1wk ahead. My waters spontaneuos ruptured 1 calender month before her due date. so 4wk3days early.

She weighed nearly 7pds at birth but she also got jaundice (more common in both preterm and induced babies/i ended up being induced) and feeding issues not uncommon for a preterm baby. She remained about a month 'behind' in her milestones up until 7mths or so where she caught up.

I didn't have an ultrsound until 12wks this baby and it has consistently measured 4-7 days a head in each ultrasound. Signs are that it will be an early baby (1/2way engaged, some mucus plug loss etc).