I don't know if its normal or not...but I'm sure if there was something really wrong your doctor would take action! My step-sister was born with a HUGE head haha and now she is an actual genius. Seriously. She graduated highschool at 16, bachelors at 20, peace corp for 2 years, masters degree at 24, and is not going to be a doctor in just a few short years (is halfway through her doctorate) and she's only 27!
So a big head is not always a bad thing but I'd watch that delivery! ouch!
Let us know what happens!

First of all congrats:)
Don't freak..your baby could just have a bigger than average head or the head is developing faster than the rest of the body. My baby's entire body is measuring a week ahead also, and my doc said it's not something they would need to investigate unless measurements are 2 weeks ahead. I'm assuming you were keeping good track of your cycle since you were ttc for quite some time, so it shouldn't be an error on your part.
I'm not personally aware of any abnormality that could cause a larger head, in fact, I want to say that down babies have smaller heads than normal. Further ultrasounds will track your babies progress, and if you want to do the blood test screening at 16 wks to put your mind at ease that may help (although they are not entirely accurate - my results landed me upset and frantic at a specialist's office and my baby looked great on their ultrasound).
Also, my son was born with a big head, in the 90th percentile, so that could be the case with your baby also.

It'll be fine :) your doctor would not lie to you.

there is a wide range in these measurements, and you are very early for them to even be measuring the head. What is most important is when you are further along for them to take many measurements and look at the overall growth. Babies don't grow in proportion, so the head may grow one day, the legs the next. I wouldn't worry about it. You can request to have the nuchal translucency test which is an ultra sound done around 12 wks (it can't be late) the tech will measure the thickness at the back of the neck and indicate if downs is a possibility. In addition to this there are 2 blood tests: one in the first trimester and one in the 2nd. *timing is important so call today and ask about it. These three tests combined give a go indication if downs & trisonomy 13 (or 18?) is a possibility. The only test that is considered 99% is an amnio, which is invasive but will be offered if the blood tests indicate a high chance of a problem. Honestly I wouldn't worry based on an ultra sound at 9wks. Your baby is sooooo tiny!

When my mom had me my head was 13centameters. My poor mother

my kids both early on measure up to two weeks further ahead. later on even more.

my second DD's head was 17 inches. only two pushes.

sorry 17 cm. her sister who was 5 weeks early had a 12 cm head.

wait i was right they measure circumference not diameter. The average head circumference (the distance around the head) is 14 inches (35 centimeters).

13inches is certainly within the norm.