Among those who delayed childbearing into their 30s or 40s and then sought fertility treatment, 40% of women “strongly agreed” they regretted the delay, according to Adachi, Endo, and Ohashi’s cross-sectional study of 449 patients across nine Japanese fertility clinics (BMC Public Health, 2020). A companion study by the same group found that 57.5% agreed — to any degree — that they wished they had decided earlier. The gap between those figures (40% vs 58%) reflects the difference between strong and any agreement; the entry uses the stricter 40% to avoid overstating the rate. Notably, the primary driver of delay regret in both studies was not age per se but lack of fertility knowledge: people did not understand that fertility declines significantly in the mid-30s, and they regretted not knowing sooner.
On the early-parenthood side, a 2025 online survey of 2,000 US parents by the Headway app found that 24% wished they had waited longer before starting a family, and 46% reported not achieving personal goals before parenthood reshaped their priorities. This is not a peer-reviewed study, but it aligns directionally with a large Danish register study (Journal of Marriage and Family, Johansen, Nielsen & Verner, 2020), which found that early parenthood — defined as before age 21 — significantly reduced educational attainment and employment for both mothers and fathers, with the catch-up period extending into the early 30s even in a Nordic welfare state with generous support.
The comparison between these two sides should be treated as directional, not precise. The inaction regret rate is drawn from a fertility-clinic sample — people who delayed and then experienced fertility difficulty — which imposes a double-selection that likely inflates the figure substantially relative to all delayed parents. Gilovich and Medvec’s framework predicts that inaction regrets intensify over time (the closed fertility window cannot be reopened), while action regrets (lost career opportunities, constrained finances in young adulthood) tend to attenuate as circumstances improve. The 16-percentage-point gap between 24% and 40% is consistent with that pattern, though the different populations and methodologies mean it should not be read as a precise estimate of the asymmetry.