'I can't believe it!' Kim Zolciak celebrates Brielle's 20th birthday... before sharing selfies of herself

She was just 18 when she became a mother. 

And on Saturday, Kim Zolciak Biermann took to Instagram to wish her oldest daughter Brielle Biermann a happy 20th birthday.

The 38-year-old reality TV star posted a photo of her mini me with a heart-warming message, sharing it from the Atlanta airport ahead of a departing flight to LA.

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The mom-of-six wrote: 'Happy Birthday to my angel @briellebiermann I just can't believe it!!! I just can't believe it was 20 years ago today I became your mom! Scared out of my mind yet so excited!

'Had I known how easy it would have been to raise you I certainly would of had a lot less anxiety! 

'I'm extremely proud of the leader you are, the heart you have and share, the love you have for your siblings, the respect, loyalty, and commitment to your family and above it all how you always have remained TRUE to you!!'

Kim ended the sweet caption with a promise that she will see her 20-year-old soon: 'Keep shining!!! Can't wait to squeeze you!! Love you oh so much!!!'

After the Don't Be Tardy star enjoyed a meal from Chick-Fil-A at the airport, she snapped a quick shot of her cute outfit in the bathroom.

Kim wore black joggers with red embellishment on either side of her shapely legs, a white cropped sweater, and blue pointed-toe pumps.

The most enviable piece of her outfit, however, was her $2million, 10-carat diamond engagement ring from Kroy Biermann, whom she married in 2011, flaunting the over-the-top stone along with a diamond-bezelled white gold watch.

She made sure to give her 2.3million Instagram followers a close-up of her hair and makeup too, posting a photo with her blonde tresses covering much of her face.

Zolciak-Biermann's pout was artfully lined and glossed and she wore dramatic false lashes.

The caption read: 'Heading to LA baby!!'

Meanwhile, Brielle spent the day prior to her birthday having her mane dyed a darker blonde in Arizona.

The Pisces beauty opted for strands of green, pink and purple within her highlighted golden locks. 

She then went on a date at an aquarium with her boyfriend, Chicago White Sox player Michael Kopech, 21, to Odysea Aquarium. 

The pitcher surprised her with a $2k yellow gold and diamond Cartier bracelet that repeated the word 'love' twice around the oval shape.

Her baseball-playing stud also took to Instagram to wish his bombshell a happy birthday, sharing a photo of the young reality star leaning back in a burgundy bikini in front of a pristine blue ocean. 

He sweetly wrote:  'Happy 20th birthday to one of the most amazing people I've ever met. 

'You're fun, kind, caring, positive, and obviously gorgeous. You've changed my life for the better since we met & I'm extremely grateful for you.

'I'm lucky to call you mine and I intend to hold onto that privilege. I love you, Brielle.' 

Since Michael is currently in training season, the couple often celebrates big events earlier or later - Valentine's Day was celebrated two days earlier to allow Michael to make it to a training session in another state. 

And Saturday morning, Brielle caught a flight to LA, where Michael had a training session scheduled at Camelback Ranch in Glendale.

While Kopech trained, she got to spend her special day with her mom.

The gorgeous mother-daughter duo met up at the airport after their respective flights, both looking as if they had just walked out of a salon and not a cramped aircraft.

Kim returned to Snapchat immediately, complaining about waiting for her baggage.

Brielle - sans coloured hair extensions - runs to her mom to show off her birthday presents from Michael.

The first thing the girls do when they get out of the airport is head to In-N-Out for a burger.