Prenatal risk ratios for Down syndrome adjust for maternal weight because maternal serum biomarker levels decrease with increasing maternal weight. This is accomplished by converting serum biomarker values into a multiple of the expected median (MoM) for women of the same gestational age. Weight is frequently not recorded, and the impact of using MoMs not adjusted for weight for calculating risk ratios is unknown. The aim of this study is to examine the effect of missing weight on first trimester Down syndrome risk ratios by comparing risk ratios calculated using weight‐unadjusted‐and‐adjusted MoMs. Findings at the population level indicate that the impact of not adjusting for maternal weight on first trimester screening results for chromosomal anomalies would lead to under‐identification of 84 per 10 000 pregnancies.