Impending parenthood is by no means the oddest subject ever tackled by a Broadway musical. But as a representative of that popular form, ''Baby'' is certainly one of the oddities of this erratic season. The new curiosity at the Ethel Barrymore Theatre deals with what columnist Walter Winchell used to call ''infanticipation.'' ''Baby'' is a sophisticated notion of a family show. In its own lighthearted fashion, it opts for the family way.
Sybille Pearson's sketchy book concerns three college campus couples of differing generations: middle-aged Arlene and Alan (Beth Fowler and James Congdon); slightly younger Pam and Nick (Catherine Cox and Martin Vidnovic); and unmarried undergraduates Lizzie and Danny (Liz Callaway and Todd Graff).
Having a baby is the couples' common concern. Feminist Lizzie resists the idea that marriage should accompany parenthood. But by the time the company reprises ''The Story Goes On'' for a rousing finale, she has succumbed to Danny's campaign that wedlock is the bedrock of family happiness. Pam and Nick, who have not yet been able to produce a child, vow to keep on trying. Arlene and Alan, who supposed that their parenting days were over, prepare to start anew.
In comically spirited fashion, and with songs by David Shire and Richard Maltby Jr. to suit a variety of natal circumstances, ''Baby'' traverses the nine months the story requires to unfold. It remains for the liveliness of the good-natured performance to overcome the almost inevitable monotony of a more than thrice-told tale. The strengths of ''Baby'' as a modest Broadway musical are reinforced by Wayne Cilento's musical staging and the strong musical direction by Peter Howard. On the negative side, lighting and amplification are relentlessly harsh.
Its producers have loaded ''Baby'' with visual gimmickry. Set designer John Lee Beatty conceals the onstage orchestra behind an atmospheric scrim - a mural of the college-town milieu. A complex arrangement of curtains, suspended from overhead tracks, swirls noisily around and about to suggest individual scenes. (The technician who operates this stop-and-go apparatus probably hasn't had so much fun since he quit playing with electric trains.) For a clinical touch, the onstage developments are punctuated by color-slide projections of an actual pregnancy in progress.