Times and social conventions change, including the concept of the family unit.

Long gone are the days when a mother tied on her apron to cook and clean while father ventured out to work, earning the money to keep their cherubic 2.4 children in school uniforms and satchels.

Today, an alarming proportion of marriages end in divorce, same-sex couples are taking part in commitment ceremonies, and career-minded single men and women are embracing parenthood on their terms.

Baby Mama is an entertaining comedy about one such corporate high-flyer and her hilarious journey of self-discovery in the company of a ballsy, trailer trash surrogate.

Written and directed by Michael McCullers, this is unabashedly frothy fun that provides plum roles for Saturday Night Live cohorts Tina Fey and Amy Poehler as the unlikely buddies.

Their quick-fire comic timing and wonderful rapport sparks the film to life, bringing a tear to the eye as the characters caper towards the delivery room. Successful Philadelphia businesswoman Kate Holbrook (Fey) has sacrificed her dream of raising a child to climb the corporate ladder at an organic food chain run by self-styled hippie Barry (Martin).

"Some women get pregnant, I get promotions," Kate tells us in voiceover, referring to her ascent to vice president of Round Earth Organic Market.

Cursed with a peculiarly shaped uterus, Kate turns to surrogacy doyenne Chaffee Bicknell (Weaver) to find her a healthy, walking womb.

Dim-witted yet lovable Angie Ostrowiski (Poehler) accepts the nine-month assignment in exchange for $100,000, to the delight of her scheming husband Carl (Shepard).

After a fight with Carl, Angie turns up on Kate's doorstep and the businesswoman gladly takes in the waif, hoping to convert the mother-to-be to a strict regime of prenatal classes and wholesome food.

As the two women contend with muddled personal lives, including Kate's romance with juice bar owner Rob (Kinnear), they sow the seeds of a most unlikely, yet touching, friendship.

Baby Mama doesn't have a single mean-spirited bone in its body as Kate and Angie wreak havoc wherever the pregnancy takes them.

The script is littered with polished one-liners ("I've just had it with all these movie stars saying, 'Look at me and my black baby!"') and Fey and Poehler make a terrific double-act, the latter bringing the house down when she has to pretend to be Kate's sister.

By focusing so strongly on his female leads, McCullers is forced to give Kinnear and Shepard's love interests disappointingly short shrift.

Baby Mama, meanwhile, delivers an abundance of hearty laughs.