Abstract Study question Are we giving teenagers in the UK a comprehensive sex and reproductive health education? Summary answer Teenagers report learning about how not to get pregnant but have little education in other reproductive health topics. What is known already Both the World Health Organization (WHO) and the United Nations Educational, Scientific and Cultural Organization (UNESCO) agree that sex education is a necessary component of education. But globally most countries teach limited information on sex education, concentrating on how not to get pregnant. They may cover puberty, contraception and sexually transmitted infections (STIs) but rarely teach how to get pregnant, PCOS, endometriosis or menopause. We have analysed the UK curriculum at the two exam stages (age 16 and 18), which has shown that the majority of topics that come under reproductive health education are not covered. Study design, size, duration We conducted a mixed-methods study, distributing an anonymous, online survey using multiple choice and open-ended questions on Qualtrics software to schools for students aged 16-18 years. The schools distributed the survey either in the classroom or the link was given to the students. The survey was live for 62 weeks (10th May 2021 – 18th July 2022). The final sample size was n = 1224. Participants/materials, setting, methods The study had ethical approval from UCL research Ethics Committee ID no: 9831/006. A total of 1244 students completed or partially completed the survey: 754 girls, 344 boys, 38 others, 17 prefer not to say. The qualitative data was analysed thematically. Main results and the role of chance Over half of the teenagers’ rated the sex education they received in their schools’ to be adequate or below (55%, 708/1080). The top three most popular topics taught in schools was contraception (90%, 968/1080), STI’s (85%, 914/1080) and puberty (83%, 899/1080). Outside of schools the most popular topics learnt by teenagers were abortion (63%, 675/1080), puberty (60%, 648/1080) and the menstrual cycle (60%, 650/1080). The majority learnt about reproductive health topics through the internet (69%, 745/1080) and social media (60%, 646/1080) and over half (74%, 800/1076) of these teenagers do not or only sometimes, talk to their parents/guardian about reproductive health topics. Common reason for not talking to their parents/guardian was because it was uncomfortable or embarrassing (46%, 386/842). Half of the teenagers (49%, 502/1024) did not know when a woman is most fertility during their menstrual cycle. Teenagers estimated the oldest age women can naturally have children as: 40 years old (10%, 104/1024), 45 years old (23%, 263/1024), 50 years old (21%, 214/1024), and the oldest age a man can naturally have children: 60 years old (12%, 122/1024), 70 years old (12%, 122/1024), 80+ years old (36%, 366/1024). Teenagers showed better knowledge of female fertility than of male fertility. Limitations, reasons for caution Only 20 schools participated so the study results are not representative of all UK schools. Personal contacts and a teacher forum were used for survey promotion. Schools in more deprived areas, faith schools and teens without internet access and a digital device did not participate in the survey. Wider implications of the findings Schools should be teaching all aspects of reproductive health. The data from this study, and school talks given by the second author has helped the development of a free teachers’ reproductive health education PowerPoints and guide which is being promoted by the ESHRE International Fertility Education. Trial registration number Not applicable