WARNING: Spoilers ahead for The Umbrella Academy season 3
How come only 16 super-powered children were born in The Umbrella Academy's Sparrow timeline rather than the usual 43? The opening scenes of The Umbrella Academy season 3 deliberately mirror season 1's opening, with a teenage girl spontaneously giving birth in public, Sir Reginald Hargreeves chasing her down, and seven children subsequently adopted as his own. Even the voice-over remains identical... more or less. Whereas the mysterious detached narrator from The Umbrella Academy season 1 uttered the line, "On the twelfth hour of the first day of October 1989, 43 women around the world gave birth," season 3 updates the quote to, "On the twelfth hour of the first day of October 1989, 16 women around the world gave birth."
The Hargreeves siblings' trip to 1963 Dallas changed the future - this much we know. Meeting Sir Reginald in the past prompted the eccentric businessman to adopt entirely different children and replace his Umbrella Academy with the Sparrow Academy, but that shouldn't directly impact how many of the children were actually born - especially such a drastic reduction from 43 to 16.
The answer lies with Harlan Cooper, or Lester Pocket, as he's now known. Viktor believed he'd removed Harlan's accidental powers in The Umbrella Academy season 2's finale, but the transference was a total failure and Harlan has struggled to tame his fearsome abilities ever since. As Harlan describes, his power connects to other super-powered children via vibrations, and is amplified by strong emotions. Few emotions could be stronger than the death of his mother, which strangely occurred on October 1, 1989. When Harlan exploded with grief, his psychic connection reached out to the future Hargreeves mothers shortly before their spontaneous children could be born and triggered brain hemorrhages in each. Harlan's massacre was completely unintentional, but seemingly prevented 27 of the 43 children being born, including all six Umbrellas he met in 1963.
Why Were Some Children Still Born In 1989?
If Harlan Cooper's mental shockwave was so powerful in 1989, where did 16 children slip through the net? Maybe Harlan simply reined in his emotions before more damage could be done, and those remaining 16 were just lucky survivors. Harlan's explanation implies that meeting Luther, Diego, Allison, Klaus, Five and Viktor in 1963 meant his vibrations were already attuned to those specific six individuals, making them easy targets for his hospital outburst, and explaining why every Umbrella mother aside from Ben's was among the 27 dead.
Another possible explanation for the 16 survivors is the mothers' proximity to Harlan on October 1, 1989. Aside from the Umbrella Academy who were already doomed by knowing Harlan personally, the remaining 21 brain hemorrhage victims might've been the 21 mothers closest to whichever California hospital Sissy Cooper died in. The 16 survivors (which includes the Sparrow Academy) could've all lived outside the blast radius.
The Umbrella Academy season 3 doesn't explicitly reveal where most of the Sparrows were born, but episode 1 at least confirms Ben Hargreeves hails from Seoul, South Korea - a good, long way from the Harlan epicenter. Distance could, therefore, be a factor in why 16 kids were successfully born. Christopher Hargreeves may provide yet another clue. The saucy cube's origin remains frustratingly mysterious in The Umbrella Academy, but it's plausible Number Seven is extra-terrestrial. If Christopher hails from elsewhere in the galaxy, Reginald Hargreeves might've needed to look beyond Earth for his last adoptive child because Harlan wiped out so many others.