Maternally Challenged

This is a not-so-rare subversion of women-as-maternal which occurs when a female character, usually a successful career gal, is revealed to know little or nothing about taking care of a baby. This will almost always be revealed by a plot in which she is forced to take care of a baby, temporarily if it's in a Series. If it's not in a series, the Maternally Challenged will probably be adopting the baby permanently. Often, other characters will just assume she knows what to do since she's a woman. But she doesn't and Hilarity Ensues.

This is Truth in Television. Humans are one of the few mammal species who do not instinctively know how to care for their offspring. Partly because it's a lot more complicated for us, what with the time they take to grow, but mostly because we are social animals that learn child care from our elders instead of having instinctive reactions. The former has the advantage of more flexible behaviour that can quickly adapt to new situations while the latter does, of course, not require any parent or social group and no learning effort, which saves the animal time and resources. Compare to Raised by Dudes, which is a rough male equivalent.

Examples of Maternally Challenged include:

Anime and Manga

In the Tenchi Muyo! episode "Hello, Baby", none of the girls in the Masaki household has a clue how to look after a baby - until Washu decides to take charge. Namely because she's done this before.
Ojamajo Doremi Sharp - how the hell would a kid know how to take care of a newborn (thankfully, the parents did help out a bit with this starting out)? Momoko went through some of the same trouble in Motto, and moreso because the other girls were now pretty experienced at this.
One of the many comedy tropes that ends up being completely not funny in Neon Genesis Evangelion is Misato's utter inability to serve as a maternal figure. She would be a passable or even effective Older Sister Surrogate if she didn't keep trying to molest Shinji and/or Asuka whenever she gets a chance.
Rosa, from Umineko no Naku Koro ni will always be BEST MOM EVER, including making her own kid willing to kill her horribly in one arc, but the clearest example of this trope can actually be found in Natsuhi, who managed to fail so horribly at raising a child that she ended up pushing it (along with some poor servant who just happened to be holding the kid) off a cliff.
Given her background and her current state, post-Eclipse Casca from Berserk actually averts this trope, as she is seen to take care of the mysterious moonchild implied to be her own child very well. Her lover, Guts, on the other hand...

Film

Diane Keaton's character in the 1987 film Baby Boom.
In Three Men and a Baby (released the same year, oddly enough), one of the men asked a girlfriend for advice on taking care of the baby, expecting that she would know what to do. She didn't.
The titular character in Enid, a biography on the life of Enid Blyton. When her baby starts crying she just stares at it, completely ignorant of what to do. In the end, she picks up the dog.
In Riding in Cars With Boys the teenage protagonist has endless trouble taking care of her new baby. She keeps getting told her maternal instinct will kick in, but she struggles every step of the way.

Literature

Happened to Alanna in Tamora Pierce's first quartet. She clearly figured it out at some point, because she now has several very healthy kids.
Mentioned in the Robotech Expanded Universe novelizations. One of the chapter quotes mentions that Max Sterling had taken over the familial and maternal roles, since his Hot Amazon wife was from a race of Artificial Humans with no concept of childbirth or childcare. When a visitor asked if she could hold the baby, Miriya threw him at her.
- The latter actually happens in Macross/Robotech, where Miriya cheerfully lobs her newborn daughter to Misa/Lisa - while Max looks on, smiling.
When Sammy Keyes is handed a shopping bag with a baby in it by a terrified teen, she has a horrible time trying to care for the baby until she can hand him over to the police the next morning.

Live-Action TV

Unsurprisingly, the title character of Bones is discovered to be Maternally Challenged in the episode "The Baby in the Bough". She got better at it by the end of the episode.
One of the early episodes of Friends had Rachel not even knowing how to hold her boyfriend's baby son. A much later episode had her not knowing what a bassinet is, at her own baby shower.
Nadira, the Big Bad's daughter in Power Rangers Time Force. The handle of a parting was handled to her by the Green Ranger, and seeing the baby was what started her Heel Face Turn. She later tries to protect a baby (maybe the same one, not sure) through the final battle between the Rangers and her dad, and seeing her almost getting hurt leads to the Big Bad's redemption.
A more serious version occurred in an episode of The Sarah Connor Chronicles, where the titular character was told by a boy she'd had to protect that she kind of sucks at being a mom. She acknowledges it in a "Workin' on it" sort of way.
Murphy Brown, full stop. She eventually settles into the role, but not without embellishing it with her own unique twists first.
Grey's Anatomy plays with this one a lot mostly with Christina in particular, recently she was put in charge of a ten year-old daughter of a patient and despite her disinterest and missteps she does form a bond with the girl. Alex actually subverts this trope by being the best with kids out of the fab five despite earlier protests that he wasn't and his Jerkass personality.
Predictably, Badass Action Girl Aeryn in Farscape expects she will fulfill this trope when her child is born.

D'Argo: You never struck me as the type who would want children. (Aeryn shoots him a significant look.) You either, huh?
Aeryn: He wants it so badly. So I do.
D'Argo: You'll come around when you see the little guy in person.
Aeryn: I hope so.

- She does, fortunately, and later says that she loves motherhood.
In the last episode of Law and Order SVU featuring Dani Beck, she develops a bond with a little girl who had been abused by her foster parents and takes her back to her apartment as a temporary living arrangement at the request of the girl. Unfortunately, this doesn't end as well as many of the other examples of this trope when the girl in question thanks Beck for her hospitality by attempting to torch Detective Beck's apartment so that they would be eternally together after they die in the resulting fire. Beck transfers out of the SVU the next morning, never to return.
- A couple seasons later, Olivia becomes the legal guardian of a young boy whose drug-addict mother abused him and was declared to be an unfit mother. This example goes much better than the Beck example as the boy Calvin is very appreciative of Olivia's efforts despite the fact she has admitted a few times throughout the series that she wouldn't make a good mother due to a number of reasons, the most frequently cited one being her job (hours, stress, etc.). The arc still ends in a bittersweet way, with Calvin being forcibly taken back to his birth mother. The final scene of the episode consists of Calvin repeatedly screaming Olivia's name as he's dragged away, with Olivia standing there, visibly upset and unable to do anything (the birth mother got an order against her).

Web Comics

PeeJee and Aubrey in Something*Positive. Before they marry, Aubrey's husband says "My mother's buying me a vasectomy for a wedding present." Even more explicitly, PeeJee says at one point "I'm not carrying a parasite for nine months for any man." Oddly enough, they adopted a baby later, and lampshade that they have no idea what they're doing, and spend most of the time talking about their new daughter's freakishly large hands. Later on, this Flash Forward's pretty hard to beat... so far. Still, at least we know the kid's going to survive to 17.
Zoe of Venus Envy is press-ganged into baby-sitting her neighbor's infant son. After the experience, she expresses some relief that the hormone treatments she takes to transition from male to female have probably made her sterile anyway.
In Drowtales, this seems to run in the Sarghress family. The matriarch, Quain'tana, is a warlord who grew up on the streets and may not have ever known what proper parenting looks like, and as a result treats her children like tools at best and trash at worst. Her daughter Mel'arnach, who probably has the strongest maternal instinct of Quain's potential heirs, is in prison and has a horrible relationship with her mother to the point of actual death threats. And Syphile, who is completely unqualified, is forced to care for Ariel since the latter was an infant. Unfortunately is this Played for Drama rather than Played for Laughs and has very real effects the lives of the characters involved.
Both of the leads from Moon Over June have an uphill battle to prove they are the reincarnation of June Cleaver; however self-described "single, perverted, promiscuous porn star" Hatsuki at least has admitted concern over her fitness to raise the child she just bore. Her OB/GYN roomie Summer (whose water broke just before she could sew up Hatsuki's c-section)?

Summer: *sigh*
Hatsuki: Let me guess, you're sad because you don't have and postpartum depression.
Summer: I was so looking forward to it! Then, bam! Nothing but happiness! Talk about a real downer!

- Not to mention Summer got pregnant not so much to have a kid, but to satisfy her pregnancy fetish. And she would have put the kid up for adoption if it had been a boy.

Western Animation

Luann from King of the Hill knew absolutely nothing about baby care in the episode where she gave birth, due to all her information being from Peggy and thus severely outdated and dangerous as a result. When Luanne and Lucky practice parenting on Bobby they make him so confused and exasperated (they literally give him mixed messages, one after another) he proclaims them to be completely useless at at it.
- Another example would be Cotton's wife Didi, who often appears to have difficulty taking care of G.H. to the point that she occasionally forgets where he is. Unlike Luann and Lucky though, it really isn't funny for two reasons. One, Didi is stuck between Cotton's sexist and rather psychotic demands and taking care of a screaming baby, which at times seems to be driving her mad. And second, since Cotton is a possibly-homicidal jackass, G.H. would have no hope for a good parent if Didi isn't up for it. Not to mention the fact that she had post-partum depression, and wasn't all together in the head to begin with.
In an episode of Johnny Test, the twin geniuses, Susan and Mary, turn themselves into babies so that their boy crush, Gill, can take care of them, much to their brother's annoyance. It doesn't take long for Johnny to find out that Gill is, in fact, a horrible babysitter who, while good intentioned, knows absolutely nothing about caring for babies and often threatens Susan and Marys' lives. Johnny steps in and proves to be a far better caretaker of his sisters as a brother should be.

Other

Cynthia Frady from the original Maddison Atkins