Katherine Heigl announces her pregnancy with the sweetest Instagram post

She says the news is 'unexpected but thrilling'

Katherine Heigl just announced her first pregnancy. The baby she and her husband of eight years, Josh Kelley, will welcome in January will be the couple’s third child and first son.

But the former Grey’s Anatomy actress is on top of it, already taking to her blog to expand on what the news means for her family.

'It was totally unexpected but thrilling,' Heigl, 37, wrote in a Thursday post after sharing her exciting news exclusively with PEOPLE. “The whole pregnancy thing is new to me but the thrill of having a baby is not.'

In the post, Heigl also shared a photo of two tiny knit booties, with the caption, 'Blue is for … BOY!!!'

Two people who are just as (if not more so) excited as Heigl and Kelley to welcome the new baby into the soon-to-be family of five? Big sisters Naleigh Moon, 7, and Adalaide Marie Hope, 4 — whom the couple adopted in 2009 and 2012, respectively.

'Naleigh and Adalaide are over the moon and [cannot] wait to spoil their new sibling rotten,' she writes, adding that of course she and her singer-songwriter husband, 36, are 'full of high hopes and bubbling anticipation.'

The family has been wanting to expand their brood, the Knocked Up star adds, but she also makes it clear they didn’t care one way or the other how it would happen.

'We were considering adoption again, fostering, or pregnancy if possible,' she shares. 'Seeing as I have never been pregnant and as my OB/GYN reminded me last year, I’m of advanced maternal age, I wasn’t sure pregnancy was even in the cards for us. Turns out it was very much in the cards!'

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'I’ve learned so much already, and know there’s even more to learn as this baby grows,' Heigl says, sharing a couple of black-and-white photos of herself wearing sunglasses while she gets an ultrasound.

'This is truly an amazingly blessed time, waiting for the child you’ve been dreaming of to make their appearance,' she continues. 'Though I’ve never been a particularly patient person, my experience of waiting for Naleigh taught me a lot in that department.

'So I will wait and eagerly anticipate, hopefully with grace, gratitude and a good attitude (ummm pregnancy hormones are real people).'

From the editors of People