Objective <p>To investigate the outcomes of pregnancies at extreme fertility age, (<i>≥</i> 45 years).</p> Methods <p>A retrospective cohort study was undertaken on women who gave birth at a tertiary center, Thailand, (1992–2022) to compare pregnancy outcomes between women of (<i>≥</i> 45 years and those of the reproductive age (20–34 years).</p> Results <p>Of 67,301 pregnancies, 121women at age of <i>≥</i> 45, as the study group, and 51,315 controls were included in analysis. The study group had a much higher prevalence of fetal trisomy (9.1% vs. 0.1%) and medical disorders. After excluding cases with abortion and severe anomalies, the rates of preterm birth (39.6% vs. 14.5%; relative risk of 2.73, 95% CI: 2.14–3.47), low birth weight (41.2% vs. 14.5%, relative risk of 2.85, 95% CI: 2.26–3.59), fetal growth restriction, preeclampsia, gestational diabetes, cesarean section, low Apgar score, stillbirth and miscarriage were significantly higher in the study group. After excluding cases with underlying diseases, such adverse outcomes were still significantly higher in the study group, for example; preterm birth rate was still as high as 36.8%. In multivariate analysis, extremely advanced maternal age remains an independent risk factor for preeclampsia, preterm birth, fetal growth restriction, and low birth weight, even after adjusting for other potential risk factors.</p> Conclusion <p>The age of <i>≥</i> 45 was at the 99.8th percentile among Thai population, implying a very close proximity to the end of reproductive life. The rates of fetal trisomy, preterm birth, low birth weight, fetal growth restriction, and other adverse outcomes were markedly higher than ever reported, even after excluding cases with underlying diseases. When compared to previous studies, the much higher adverse outcomes were likely caused by more advanced fertility age despite the same chronological age.</p> Clinical trial number <p>Not applicable.</p>