The exact mechanisms by which aspirin prevents the development of preeclampsia in high-risk patients are currently not fully known. Furthermore, a small proportion of high-risk patients who are on low-dose aspirin (LDA) still go on to develop preeclampsia (PE). This longitudinal observational study will assess the immune profile in participants who are taking low dose aspirin (LDA) in pregnancy. As part of routine care, patients at high risk of developing preeclampsia are treated with LDA from 16 weeks gestation. The study will be conducted at Barts Health National Health Service (NHS) Trust. The study population will comprise of 2 groups of participants: 1. Those who respond to LDA and do not develop preeclampsia (responders) 2. Participants who do not respond to LDA and develop preeclampsia (non responders) Participants will be consented at their booking appointment. Participants will be eligible if they have a singleton pregnancy and are aged over 18 years. They will have an additional blood sample taken at 12, 20, 28 and 36 weeks gestation. The blood samples will be tested to assess immune cell function, metabolism and genetics. This will identify cumulative changes in immunobiology at key time points in pregnancy.