The authors present a longitudinal study with a large sample size assessing the association between pregnancy and the onset or progression of myopia. The study participants included the SUN cohort which comprised of young adult female university graduates in Spain. Recruitment of participants started in 1999 and is permanently open. All participants included in the SUN cohort have university studies and over 50% are health professionals. The effective sample size for this analysis was 10,401 women; including only women between 20 to 50 years (mean age at baseline: 32.9 SD: 8.1). Every woman was followed up for a minimum of two years and a maximum of 14 years; 3180 females reported pregnancy during this period. Participants were considered a case of myopia when they reported a new diagnosis of myopia or an increase in myopia of at least −0•50D in one eye, as assessed by a qualified ophthalmologist or optometrist. Pregnancy was inversely associated with the risk of myopia development or progression, with fully-adjusted HR: 0.58 (95% CI: 0.49–0.69) after adjusting for known potential confounders. An adjusted HR was estimated after a multivariable analysis stratifying by age and the inverse association remained statistically significant, suggesting that the inverse association between pregnancy and myopia was not related to age: HR: 0.59 (95% CI: 0.47–0.76) in women under 30 years of age, and HR: 0.64 (95% CI: 0.43–0.97) in women older than 30 years. This inverse association was more evident in younger women, but the P-value of the product-term for statistical interaction was not significant (P=0.68). A likely explanation of the inverse association between pregnancy and myopia progression is the higher exposure to outdoor activities in pregnant women during their maternal leave. Time of outdoors physical activity of 6.9 vs. 5.2 h per week (P=0.002) in pregnant woman compared to non-pregnant woman. The study showed that pregnancies have a protective effect on myopia onset or progression. A lower risk of myopia onset or progression was found in women with one pregnancy (HR=0.55; 95% CI: 0.34–0.89), HR of 0.72 (95% CI: 0.48–1.07) was found in women with two pregnancies, and the risk was for those who referred three or more pregnancies was HR: 0.43 (95% CI: 0.24–0.75). Limitations of the study include: Inclusion of self-reported data, a biased cohort design of highly educated participants, mostly Caucasians, lack of information on pre-existing myopia, lack of information of variables such as parental history of myopia, or types of near work and profession. Also, the selective bias of the likelihood between a pregnant and non-pregnant woman visiting for an eye check was not considered.