Introduction: Fertility rates in the United Kingdom have remained below replacement level for more than a decade, reflecting broader patterns of delayed childbearing and smaller completed family sizes in high‑income countries. While structural and economic determinants of fertility have been widely examined, less is known about whether interpersonal social support influences actual fertility outcomes specifically, the number of children women have within the UK. Most existing studies focus on fertility intentions rather than completed family size, leaving an important evidence gap. Methods: This retrospective cross‑sectional study used secondary data from Waves 1–14 (2009–2024) of the UK Household Longitudinal Study (Understanding Society). The analytic sample included women aged 18 and over with at least one biological child (N = 822 complete cases). Perceived social support from spouse/partner, family, and friends was assessed via single‑item survey questions and recoded into binary variables. Demographic covariates included age, ethnicity, highest educational qualification, net monthly personal income, and overall life satisfaction. The number of biological children was modelled as a count outcome using Poisson generalised linear regression with a log link. Ethical approval for secondary data analysis was obtained from the University of Huddersfield School Research Ethics and Integrity Committee, and all procedures complied with UK Data Service End User Licence requirements. Results: This study found that age was the only statistically significant predictor of women’s number of biological children. Each additional year was associated with a small but significant reduction in expected child count (IRR = 0.987, p = 0.002). Perceived support from spouse or partner, family, or friends showed no statistically significant association with number of children after adjustment. Income, education, ethnicity, and life satisfaction were also non‑significant. Conclusions: Among UK mothers, life‑course timing as captured by age was more strongly associated with the number of children than current perceptions of interpersonal social support. These findings highlight the importance of timing, measurement alignment, and distinguishing between intentions and outcomes. Future research should employ longitudinal, parity‑specific approaches to better understand how interpersonal and structural factors interact to shape fertility behaviour.