Sure, Katherine Heigl and Seth Rogen are nervous parents-to-be in "Knocked Up," but that's a normal state even when the pregnancy isn't the result of a drunken one-night stand. The movies have offered up far more stressful gestation periods, as our list of the top 10 most nerve-racking pregnancies in film shows.

10. "The Fly" (1986)

A lesson in safe sex if ever there was one: Science reporter Veronica (Geena Davis) enters a whirlwind relationship with wacky telepod inventor Seth Brundle (Jeff Goldblum) and soon finds herself pregnant. The problem is, during a teleportation test, Seth's DNA merged with that of a housefly, causing him to mutate into an oozing, wall-crawling, vomit-dropping fly/man hybrid. Ronnie's obviously concerned that the baby will have his father's eyes (and wings and proboscis), prompting a childbirth nightmare that's even more disturbing than, well, actual childbirth!

9. "The Blue Lagoon" (1980)

The third adaptation of the 1908 romance novel casts the sun-toasted Christopher Atkins and Brooke Shields as Richard and Emmeline, the eventual sole survivors of a shipwreck that leaves them on a South Pacific island in the 1800s. With no Internet access, Dr. Phil episodes or even smart apes to raise them properly, they must traverse the murky waters of puberty in ignorance, and, as one thing leads to another, Emmy soon wonders why she's getting so fat. It only takes her nine months to figure it out.

8. "Rabbit Test" (1978)/ "Junior" (1994)

It's not a common movie debate, but we'll bring it up: Which "pregnant man" movie is the worst? In "Rabbit Test," Billy Crystal plays Lionel Carpenter, a loveless night-school teacher whose first sexual encounter leaves him mysteriously with child. This laboriously unfunny misfire should remain out of print forever. Let us all give thanks that Joan Rivers' first directorial effort was also her last. The pregnant-papa plot got another go 16 years later in Ivan Reitman's "Junior," this time with Arnold Schwarzenegger as the miracle man in what the studio thought was surefire comedy gold. Again, not really. An attempt to give scientific plausibility to the pregnancy is just one of this film's many mistakes. Ahnuld's manic mugging has no reins, and the only saving grace is that his C-section spares the audience his full-on labor pains. In all fairness, there have been other movies on this subject, including the 1973 French farce "A Slightly Pregnant Man," and as far as we can tell, none of them have been funny. Maybe the world's just not ready yet. But speaking of the governor of California ...

7. "The Terminator" (1984)

Many expectant parents have delusions of grandeur, believing that their unborn children will grow up to save all of humanity. But in James Cameron's now-classic sci-fi actioner, that's literally the case. Sarah Connor (Linda Hamilton) can't figure out why she's being chased by a gun-toting Schwarzenegger until she meets the mysterious Kyle Reese (Michael Biehn). Reese convinces her that the cybernetic Terminator has been sent from the future to kill her before she gives birth to a son who will lead mankind in a resistance against intelligent machines that have taken over the world. Reese knows this because he's also from the future, sent by John Connor to save his mother, and he takes his job seriously enough to be the guy to impregnate Sarah with the heroic seed. Time to stock up on prenatal vitamins!

6. "It's Alive" (1974)

In its review, The New York Times warned that this film contains "a scene of childbirth that's grizzly [sic] enough to put anyone off both motherhood and fatherhood." They should've added "and obstetrics," as the mutant baby that springs forth from its mama slaughters everyone in the delivery room before setting off on a swaddling killing spree. This is the only film on our list to deal with the anxiety the baby feels as it leaves the warm amniotic embrace of the womb for the cold, cruel world we enter with a slap and a slice. Brrrr!

5. "Alien 3" (1992)

After doing battle with the gooey murderous species in the first two "Alien" films, Ellen Ripley (Sigourney Weaver) had come to know the skull-splitting creatures intimately -- too much so. In David Fincher's oft-maligned third entry in the series, Ripley finds herself the sole survivor of "Aliens," stranded on an all-male planet/ mining facility. Distraught over losing the surrogate daughter she gained in "Aliens" to the beasts, Ripley then discovers to her horror that she's gestating one herself. Not exactly the kind of motherhood she had planned on, so she leaps into a river of molten metal at the precise moment of "birth" as the baby alien bursts through her chest.

4. "Agnes of God" (1985)

In this philosophical/ theological mystery movie, a cloistered novice nun, Sister Agnes (Meg Tilly), has just killed the baby she claims was divinely conceived. Or was it rape? If so, how and by whom? And if it was an immaculate conception, doesn't that make Sister Agnes' crime, um, really bad? Crusty psychologist Martha Livingston (Jane Fonda) is brought in to investigate, and she ends up clashing with Mother Superior Ruth (Anne Bancroft) over the often-at-odds ideologies of faith and science. But not over contraception -- that's not up for debate.

3. "Dawn of the Dead" (2004)

In Zack Snyder's remake of George Romero's classic 1978 zombies-in-a-mall flick, Mekhi Phifer plays Andre, an expectant father whose doting on his baby's mother, Luda, becomes even more frantic when she's bitten by a zombie. Andre ties Luda down and tries to keep her alive until she gives birth. Bad news for Andre comes in threes: The other refugees have discovered his secret, Luda dies and becomes a zombie before giving birth, and his son takes after mommy!

2. "Children of Men" (2006)

After a mysterious virus renders the entire world infertile, one lone pregnant woman represents the only hope for the future. Clive Owen plays Theo, the man given the thankless job of shepherding the pregnant Kee (Claire-Hope Ashitey) through a war-torn world that's bleaker than a Radiohead record and into the welcoming arms of the Human Project. Let's just say that it takes the stranger-rubbing-your-belly invasion of personal space to a whole new level.

1. "Rosemary's Baby" (1968)

One of the most irritating things about being pregnant is suffering the interference of all the people who think they have wisdom to impart. Never was this more the case than in Roman Polanski's thriller, in which the expecting young Rosemary (Mia Farrow) is fed herbal tea and snacks by the doting old couple next door. When Rosemary begins to suspect ulterior motives of the Satanic kind, her husband tries to have her committed. The crazy thing is that Rosemary's right. But maternal instinct is a powerful thing, and she can't turn her back on the child when it's born ... even if Satan is the baby daddy.

Who would have thought that pregnancy could manage to inspire such frightening horror films and comedies rooted in fear and ignorance? After all, the "miracle of childbirth" is an occurrence so common that it happens worldwide approximately every second.

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