Background Pregnancy is associated with deteriorations in maternal bone strength and heightened susceptibility to bone fractures. We aimed to investigate whether ambient particulate matter (PM)(2.5) concentrations were associated with bone strength during pregnancy. Methods In this longitudinal cohort study, we analysed longitudinal data from women participating in the Programming Research in Obesity, Growth, Environment and Social Stressors (PROGRESS) cohort in Mexico City, Mexico. Eligible women were aged 18 years or older, at less than 20 weeks' gestation at the time of recruitment, planning to stay in Mexico City for the next 3 years, without heart or kidney disease, did not use steroids or anti-epileptic drugs, were not daily consumers of alcohol, and had access to a telephone. Daily ambient PM2.5 concentrations were estimated from a spatio-temporal model that was based on the individual's address. Trabecular bone strength was measured using quantitative ultrasound from the radius of the middle finger and cortical bone strength from the proximal phalanx of the middle finger, during the second trimester, third trimester, and 1 and 6 months post partum. Bone strength T scores were modelled with PM2.5 concentrations using linear mixed models and distributed lag models. Findings Adjusting for multiple exposure windows, each 10 mu g/m(3) increase in PM2.5 exposure concentrations in the first trimester was associated with a 0.18 SD decrease (95% CI -0.35 to -0.01; p=0.033) in ultrasound speed-of-sound (SOS) T score of trabecular bone strength from the second trimester until 6 months post partum. Similarly, each 10 mu g/m(3) increase in third trimester PM2.5 exposure was associated with a 0.18 SD decrease (-0.36 to -0.01; p=0.044) in the SOS T score of trabecular bone strength from the third trimester until 6 months post partum. PM2.5 exposure in the first month post partum was associated with a 0.20 SD decline (-0.39 to -0.01; p=0.043) in cortical bone strength until 6 months post partum. Interpretation Ambient PM2.5 exposure during and after pregnancy was associated with diminished trabecular and cortical bone strength. Early pregnancy PM2.5 exposure was associated with a greater decline in bone strength later during pregnancy. Late pregnancy and early post-partum exposures adversely affected the post-partum bone strength recovery. Technological and policy solutions to reduce PM2.5 pollution could improve public health by reducing bone fracture risk.