It's a.....

 Boy!  
Nicholas is thrilled. Ellie is a bit disappointed but mostly indifferent, and catching on to the excitement by the minute.
I am surprised. I was certain it was going to be a girl this time.  I knew I'd be delighted either way, but since I was right about the first two's genders, I thought I knew for sure this time. 
But nope, we've got another darling little boy headed our way,
and I couldn't be happier.

We took the kids to the ultrasound. It was so fun, so happy. Will probably become one of my favorite moments of my life, just as seeing my other babies in the ultrasounds have been. This is when it becomes real. Baby brother even waved at the kids partway through.  

And because we're keeping it real here, 

 After the delight about the baby being a boy, Nicholas proceeded to throw the biggest meltdown tantrum about his balloons. Strings were too long. Then not long enough. The bow I tied on his wrist wasn't right. Then it came untied. Then he decided the balloon Ellie had was meant to be his...and so on.
Overexcitement?  Perhaps.

We just can't wait to meet you, little brother!
Also, I guess "Girl-O" is out...