Pregnancy is a critical period for health investments, bolstered by extensive public health guidance and evidence that healthy prenatal environments foster child development with lifelong benefits. We provide new evidence on the impact of conception on nutritional choices during pregnancy and infancy, using an event study and household scanner data. Households make considerable healthy adjustments in alcohol and fruit purchases. Yet, despite these, the nutritional quality of food choices, measured using the FSA’s nutrient profiling model, declines substantially during pregnancy. The magnitude of the decline amounts to 50% of the mean difference in nutritional quality between a normal weight and an obese individual. It is caused largely by increased purchases of ultra-processed, high-sugar foods currently in the focus of food regulation. The deterioration of nutritional quality varies little by socio-economic status, pregnancy risk factors and food price environments, resulting in a fairly universal in-utero exposure to less healthy food environments.