Abstract Study question Do health and lifestyle behaviours from 300,065 UK-based women indicate preconception choices exceeding national recommended limits? Summary answer The majority indulged in lifestyle behaviour(s) that negatively impact fertility despite actively trying to conceive or wanting to start a family in the future. What is known already Existing knowledge underscores the impact of alcohol consumption, drug use, smoking, and vaping on preconception health and fertility. Understanding the associated risks is crucial for optimising reproductive outcomes. Awareness regarding limits of exposure and intake also remains low, hindering informed decision-making. This study, examining data from 300,065 UK-based women, aims to bridge this gap by shedding light on the prevalence of lifestyle choices and their potential implications for fertility. Emphasising the need for heightened awareness, the findings can contribute to informed preconception counselling, promoting healthier lifestyle choices and empowering individuals with the knowledge needed for optimal reproductive well-being. Study design, size, duration A cross-sectional study was conducted on 300,065 women aged 18-58 who used an at-home hormone testing service between September 2020 and January 2024. Users were asked to complete an online health assessment (OHA) assessing demographics, menstrual cycle history, lifestyle and medical history and asked to select one or more categories that best described them - planning for the future (PFF), actively Trying to Conceive (TTC), just curious about their reproductive health, concerned only about symptoms. Participants/materials, setting, methods Among the 300,065 users, 133,682 were PFF, 76,692 were TTC, 81,257 were just curious, and 42,966 were only concerned about symptoms. We analysed the lifestyle habits of users including alcohol consumption, smoking, vaping and recreational drug use. Only users who were PFF, TTC or just curious were included in the analysis. Counts, means and percentages of completed VHAs were calculated using R software. Main results and the role of chance For those TTC, the mean age was 30.7, 39.53% (n = 30,317), consumed 1-14 units of alcohol weekly, 3.95% (n = 3,028) exceeded 14 units, 21.95% (n = 16,834) regularly or occasionally smoked, 22.40% (n = 9,436) regularly or occasionally vaped, and 7.06% (n = 5,415) used recreational drugs regularly or occasionally. Among those PFF, the mean age was 28.3, 57.27% (n = 76,556) consumed 1-14 units of alcohol weekly, 5.31% (n = 7,095) exceeded 14 units, 22.86% (n = 30,566) regularly or occasionally smoked, 27.29% (n = 21,002) regularly or occasionally vaped, and 13.52% (n = 18,071) used recreational drugs regularly or occasionally. For those who were just curious, the mean age was 27.9, 58.10% (n = 30,602) consumed 1-14 units of alcohol weekly, 5.70% (n = 3,010) exceeded 14 units, 21.17% (n = 11,143) regularly or occasionally smoked, 28.53% (n = 15,018) regularly or occasionally vaped, and 14.63% (n = 7,701) used recreational drugs regularly or occasionally. Limitations, reasons for caution As all data was self-reported, there is a risk of recall bias and inaccurate reporting. As the users who completed the OHA were seeking a reproductive healthcare service, there is likely a bias in the selection criteria of study participants. Wider implications of the findings Although those TTC demonstrated fewer adverse lifestyle behaviours our data demonstrates that a significant number were still not following national recommendations. This highlights the need for tailored preconception education to improve fertility awareness and uptake of practices to improve preconception health and pregnancy outcomes. Trial registration number NA