Thirteen Times More

The largest study of its kind, published in 2020 in Human Reproduction, followed 312,656 women born in Norway and compared two generations: those born between 1936 and 1939, and those born between 1960 and 1964. The gap between them shows exactly where reproductive life actually moved — and where it barely moved at all.

The gap between the two generations is small at one end and large at the other. Age at first period fell from 13.42 to 13.24 years — 0.18 years, roughly two months, across twenty-five years of generations. Age at menopause rose from 50.31 to 52.73 years — 2.42 years directly, a shift the authors’ own summary describes as “nearly three years.” Dividing the two, 2.42 by 0.18, menopause moved roughly thirteen times further than menarche over the same span.

The reproductive window — the interval between the two events — widened from 36.83 to 40.22 years, a 3.39-year increase. That figure doesn’t fall out exactly from subtracting the two rounded averages above (that gives 2.60 years): the authors calculated it for each woman individually, then averaged, which yields a slightly different result than subtracting the rounded means directly. The gap between the two methods just shows how sensitive rounding is at small numbers, not an error.

Why the two ends don’t move together

Menarche has, biologically, a threshold. A girl’s body begins puberty only once the hypothalamus registers that energy reserves are sufficient for a future pregnancy. In the 1970s, researcher Rose Frisch proposed a simple version of this idea: a fixed threshold of roughly 17% body fat to trigger menarche, and 22% to sustain fertility. Today’s picture is more nuanced — leptin, the hormone produced by fat tissue, plays a permissive role within a broader network of metabolic signals, not the role of a single switch. But the practical conclusion holds: once a population’s nutrition crosses a sufficient level, the threshold gets reached earlier and earlier — up to a point. Past that point, there’s nowhere left to fall. That’s exactly what the Norwegian figures show: the decline has nearly stopped.

Menopause has no equivalent threshold, crossed once. It reflects the gradual depletion of the ovarian follicle reserve, a process sensitive to health across an entire adult life — smoking, chronic disease, access to medical care — not just childhood nutrition. That’s the source of the asymmetry: one generation can reach the puberty threshold almost as fast as the one before it, yet keep gaining years of adult health long afterward, and those years are what push menopause later.

One threshold, three speeds

A separate reconstruction, published in 2025 in Annals of Human Biology from 783,757 observations across 33 studies, traces the age at menarche in Norway from 1840 to 2008 and finds three distinct speeds, not a steady decline.

The fastest pace doesn’t belong to the generation that grew up during the prewar economic boom — it belongs to the one born during the war itself. The likely explanation: girls born then didn’t spend their childhoods amid wartime shortage; they grew up through the postwar nutrition and public-health boom after 1945 — their childhood, not their birth, coincided with the recovery. After 1950, the pace drops to 0.06–0.08 years per decade, close to the 0.18 years over 25 years measured directly above — a sign that two datasets, built by different teams with different methods, converge on the same conclusion.

The 150-year benchmark

For scale, the classic study by Gail Wyshak and Rose Frisch, published in 1982 in the New England Journal of Medicine, pools data from across Europe and shows a decline from 15-17 years, among women born in the early 1800s, to 13-13.5 years, among those born in the 1950s — an average of roughly 2-3 months per decade over 150 years. The Norwegian figures above aren’t added directly to this benchmark, since they come from different sources and populations, but they fit the same story: a long descent that has drawn close to a floor.