Alyson Hannigan showed off her growing baby bump in a dark blue sweater, plaid mini skirt and black tights while on a girls-only shopping trip with daughter Satyana in Santa Monica, on Monday afternoon.

Hannigan has had a very busy month promoting the upcoming flick American Reunion, while also planning a joint birthday celebration with her adorable 3-year-old -- they both share March 24 birthdays!

The How I Met Your Mother star recently told Celebrity Baby Scoop on her big bday plans, "I've been trying to figure out the best birthday party, but I think we might actually just do a trip to Disneyland with a few close friends instead of a big party. I think that would be really special for her and for all of us."

Alyson Hannigan and hubby Alexis Denisof picked up some party supplies in Santa Monica Friday ... in the form of some HUGE, brightly-colored balloons.

Little Satyana Marie's third birthday isn't until the end of the month, but perhaps the family was getting a jump on the big day? Or maybe the smiley-faced sunshines and rainbow-colored butterflies were just an accessory to brighten their day!

The How I Met Your Mother star announced her second pregnancy back in December so she could be due around mid-May, assuming she announced at the beginning of her second trimester. In fact, her character on the show is due in May, so perhaps producers are writing the show in real-time!

Hiding her baby bump under a pair of relaxed jeans and a sweater, Alyson Hannigan enjoyed a fun-filled day of running errands in Beverly Hills with daughter Satyana on Tuesday The little tyke even picked up a big red balloon after getting some new shoes.

The darling 2-year-old has been over the moon since learning she's set to become a big sister, and the little girl already has a few names in mind for the baby-to-be!

Alyson shared via Twitter on Tuesday, "My daughter said 'Mommy I have great names for the baby in your belly! If it's a girl, Coco. If its a boy Gaguzzi. It sounds like Jacuzzi!'"

Armed with a miniature stroller and baby doll, Alyson Hannigan's two-year-old daughter Satyana looked like a proud big sis-in-training, as the pair stepped out in Santa Monica Thursday afternoon.

The How I Met Your Mother star just announced her pregnancy last month, but she's really starting to show, and her little girl is defintely taking notice! Aly Tweeted this week, "This morning my daughter said to me 'Wow Mommy, your tummy is getting BIG!' I'm really glad I'm pregnant & she didn't say butt."

As for the sex of baby No. 2, Hannigan also Tweeted: "All I want for Xmas is to find out the gender of the baby in my belly! ... I really don't have a preference I just want to stop calling it 'it'!!!"

Alyson Hannigan and hubby Alexis Denisof are expecting!

The How I Met Your Mother star showed off her tiny baby bump while on a stroll with 2-year-old daughter Satyana in Santa Monica.

A source close to the 37-year-old actress tells Us:

"Alyson is thrilled to be pregnant! She's been wearing baggy clothes to cover the baby bump."

As for how her pregnancy will affect her role on HIMYM, Alyson lucks out because her character, Lily, is also expecting on the show! Talk about perfect timing.

See pix of Hannigan's baby bump in our photo gallery below!

The whole American Pie gang is back for the new flick American Reunion!

Jason Biggs, Alyson Hannigan (and her flute), Seann William Scott, Natasha Lyonne, Thomas Ian Nicholas, Tara Reid, Mena Suvari, Eddie Kaye Thomas, Eugene Levy and Stiffler's mom Jennifer Coolidge are ready to make us laugh once again with raunchy jokes delivered oh-so-well. Even the luscious Shannon Elizabeth makes a cameo!

American Reunion hits theaters on April 4, 2012.

All dolled up as Mr. and Mrs. Scarecrow, Alyson Hannigan and hubby Alexis Denisof held on tight to their little pumpkin Saytana as they headed to a Halloween party, before taking part in some neighborhood trick-or-treating, in Brentwood.

The How I Met Your Mother star outfitted her darling 2-year-old in an adorable Jack-O-Lantern costume, complete with matching candy pail, black tights and orange Crocs.

What a sweetie!

The cast of 1999's American Pie have reunited, and it feels (and looks) so good!

In this photo, original cast members Jason Biggs, Tara Reid, Mena Suvari, Sean William Scott, Alyson Hannigan and Thomas Ian Nicholas are striking a familiar pose for the upcoming film American Pie Reunion.

As for the vibe on set, Sean William Scott tells People:

"In many respects we've grown up a lot. But on the other hand we haven't grown up at all. On-set, Jason Biggs was farting up a storm, Chris Klein was belching, and Eddie Kaye Thomas and Biggs were hitting each other in [the groin]."

Some things never change! American Pie Reunion hits theaters on April 6.