As its title suggests, the latest episode of "Up All Night" takes us back 18 months to the events surrounding little Amy Brinkley's introduction to the world. The poop jokes are in place, as is the requisite birthing video-viewing-turns-to-horror scene. As an episode of a comedy series, it's not the strongest installment, but as a device to make viewers care more about the characters and their relationships, it works quite well. And shucks, it's just sweet.

"Birth" also helps to paint Ava in a more sympathetic light. Sure, she's a self-obsessed freak, but she's also a lost little girl ever since her best friend grew up and took the path to domesticity. (Interestingly, Maya Rudolph and Christina Applegate's relationship on the show is quite similar to that of the leads in "Bridesmaids," with Applegate in Rudolph's role and Rudolph playing Kristin Wiig's part.) Before Amy was born and threw a big ol' wrench into Ava's friendship with Reagan, Ava tried to duck that wrench with denial, attempting to simply ignore that Reagan had a human being growing inside her. Determined to help Ava just suck it up and accept the change was Missy, who offered to make a big change of her own: finally cutting the Crystal Gayle-length hair that she'd clung to since middle school.

Reagan, on the other hand, hasn't changed much at all. Pre-baby, she was still a control freak who actually wrote up an 18-page birthing plan, covering every detail of how the blessed event should go down, right down to the magical headband that was to keep her from looking like too much of a sweaty mess. Of course, when the plan began to unravel, so did she, and it was up to Chris to convince her that everything would be OK. (And how could you not trust a guy who thinks to bring a stapler to the hospital?)

As for Chris, what a difference 18 months makes. He was thisclose to being named partner at his former law firm before Amy was born. Since the pilot, I had been expecting some big revelation that led Chris to just abandon his career, but it almost seemed like a purely emotional knee-jerk reaction to watching his new baby sleep in his wife's arms. (Well, there was also a brief mention of a dad who was never around.) While such a quick turnaround may not be all that interesting (or entirely believable) plot-wise, it's actually preferable to some hackneyed storyline about Chris realizing that law wasn't for him when he screwed over the little old lady who slipped on an icy sidewalk in front of the library. And really, Chris's willingness to give up his job to be a full-time daddy is totally in character; he's a white (and nerdy) knight whose armor just gets shinier with each episode. 

Refining all of the main characters seems to be the mission of "Birth," and in that it succeeds. Ava is, was and will always be crazy, but she's also a good friend who can occasionally stop thinking about herself and come through. (Yeah, she came bearing weed the first time, but eventually bought Reagan a beautiful headband.) Reagan is, was and will always be a Type A personality, but motherhood is slowly making her realize that she can't always be in control. Chris is, was and will always be Reagan's rock. Maybe he was more aggressive and ambitious back in his lawyer days, but in his office flashback, he didn't exactly look comfortable in that $5,000 suit. (Come on!) Certainly poring over stacks of evidence and using a bunch of Latin terms appealed to Chris's geeky nature, but he's clearly always cared more about being a good man than having a good job.

Now that we understand the characters a little better, it's time to put away the Wayback Machine. An occasional trip to the past is fun (especially when there's a Kelly Bundy sighting), but now it's time to focus on development—Amy's and the overall story's.

Grade: B

Random Thoughts:

• I kind of wish that Ava's former announcer, Barry, was still on the show. I loved how he drowsily spelled out website names, complete with the http and backslashes. I'm still not sure what they're doing with Nick Cannon (Calvin).

• Speaking of the supporting characters, do we really need Missy? Despite having one pretty funny line on this episode, I don't think she adds anything. But I can't tell if it's just a poorly-written character or if Jennifer Hall is just not a very good actress. 

• Continuing to speak of supporting characters, it was wonderful to bump into Gene and Terry at the hospital and to discover that—ugh!—they've always been terrible.

• I once spent five days in the hospital and wasn't able to shower or shave. This was at the NYU Medical Center, where apparently all the doctors are also models or actors. So, I totally felt Reagan's pain about not wanting the hot doctor all up in her business when things got real.

• That nurse who stopped Chris from taking off his clothes? Fire her IMMEDIATELY!!

• This show continues to reference all the best alternative music...from the '90s.

• Best quotes:

Ava: "This baby is gonna blow at any second. There will be cervix everywhere!"

Missy: "Listen to me, b-word. Get your sweet a-word in the car. You're going to help your friend. F-word."

Reagan: "We drank a $9.00 bottle of wine and now there's another human being on the Earth."