The estimate comes from Japanese work on specimens from Suruga Bay, where large numbers of frilled sharks are caught incidentally. Rather than observing a pregnancy, researchers measured embryo lengths across many females and divided the size range by the growth rate, which yielded roughly forty-two months.
The pups develop on yolk alone with no placental connection, in cold, food-poor water where slow development is affordable. Litters are small, typically two to a dozen, and the species appears to breed without a season, since there is no annual temperature or light cue at depth.
The figure is an inference, not a stopwatch. It rests on the assumption that growth is linear and equal in all females, and no frilled shark pregnancy has ever been followed from start to finish. Comparable claims for other deep-sea sharks remain scarce for the same reason.