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In “Some Luck,” the first book in Jane Smiley’s ambitious planned Last Hundred Years trilogy, Rosanna Langdon delivered her fifth baby, Henry, completely alone, howling in an isolated Iowa farmhouse while husband Walter tended the land, out of earshot among rows of corn.

What better marker of the generational changes that have shaped the Langdon family in the most recent installment, “Early Warning,” than another home birth scene — this one taking place in the upper reaches of a New York apartment building 50 years later. When Loretta Langdon, the wife of one of Rosanna’s many grandsons, feels her water break amid dinner party chatter about the sinking price of crude oil, she doesn’t grab onto the nearest bedpost and commence to praying but calmly climbs into the bathtub, attended by an editor from a major publishing house who “worked on a book a couple of years ago about non-violent birth and baby massage.” We’re definitely not in Iowa anymore.

When, in “Some Luck,” the younger Langdons escaped the farm for college or family life, it was impossible not to root for them. Yet the trilogy’s second entry loses a unifying force in the somber primacy of the Iowa farmland. Though second son Joe stays in Denby to expand and modernize the farm, “Early Warning’s” opening scene in 1953, the funeral of patriarch Walter Langdon, signals the end of its centrality to the narrative.

The only motif shared by the next 33 chapters, each of which covers one year, is the vague, Cold War-inflected dread alluded to in the title. Thrill-seeking World War II vet Frank lives with his chic wife Andy and troubled kids in a New Jersey mansion; Lillian and her CIA spook husband Arthur raise a brood of four in the suburbs of D.C.; academic Henry lives as a closeted gay man; and the youngest Langdon, Claire, stumbles into a loveless marriage with a germaphobe Des Moines doctor.

As the children multiply, so, too, do the perspectives. Smiley’s free-indirect narrator jumps in and out of all the characters’ heads at will, from babyhood onward. The breadth obtained is dazzling, but with this many Langdons — the family tree in the front of the book names more than 60 — we never get to spend much time with any one character. Although “Early Warning” is substantially longer than “Some Luck,” the emotional stakes feel lower. It’s like leafing through a gigantic family album, pausing to hover briefly over each snapshot, then turning the page.

This is by design. Smiley is less invested in the rapid ups and downs of individual life than in the broad, slow sea changes of American life itself, refracted through one American family. Thus, even dramatic scenes like the two births rarely turn out for the worst; tragedy waits for quieter moments, sneaking in unexpectedly and settling in for a lifelong stay.

Smiley is particularly good on the kind of family dynamics that simmer along for decades just under the surface, never quite coming to a head. For instance, Frank’s prematurely born twins, Richie and Michael, are too small and weak for his taste; to make up for it, he pits them against each other relentlessly, even setting up a home race course for them at 2 years old and encouraging each to beat the other at any cost. Though inseparable, the boys grow up secretly hating each other, expressing their sociopathic tendencies in dangerous pranks. But where readers of Smiley’s Pulitzer-winning rewrite of “King Lear,” “A Thousand Acres,” might expect a fratricide down the line, the twins’ antipathy mellows in adulthood to a mild prickliness. Shakespeare this isn’t.

Meanwhile, the ’50s turn into the ’60s, and then the ’70s, with close calls but few real blow-ups. One character becomes obsessed with increasingly baroque forms of psychoanalysis; another dies in Vietnam; still another becomes involved with Jim Jones and the Peoples Temple. It’s hard to strike a balance in this type of book between everyday life and the touchstone events that mark all members of a generation, and Smiley occasionally embroiders the periphery of the story with encounters that feel superfluous. Why saddle a Langdon cousin who vanishes after the first few chapters with an affair with Beat icon Neal Cassady? And Lillian’s husband, Arthur, isn’t just any spy, but a top man, taking orders from real-life presidential advisers Frank Wisner and McGeorge Bundy. The name-dropping weighs the story down and has the unintended side effect of raising questions about what’s left out — the civil rights and women’s movements, for example, are scarcely to be found, and the rise of computers happens offstage.

But with just one chapter to cover each year in the lives of dozens of characters, prioritizing is a given. Besides, there’s the final book yet to come. If it continues the conventions established by the first two, we’ll follow Langdon descendants all the way through 2020, where birth scenes may come to us via status updates.

Amy Gentry is a freelance writer living in Austin, Texas. She has a doctorate degree in English.

“Early Warning”

By Jane Smiley, Knopf, 481 pages, $26.95