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Excessive release of syncytiotrophoblast vesicles (STBM) from the placenta into the maternal circulation may cause the inflammatory response, endothelial dysfunction and activation of the coagulation system characteristic of pre-eclampsia (PE). Consequently, other cell types including platelets, leukocytes, red blood cells (RBC) and endothelium may be activated to release cellular vesicles which exacerbate the disease. This thesis aimed to develop methodology for enumerating and phenotyping STBM and the other vesicle types to determine whether they could be used as biomarkers for PE. In vitro derived STBM and vesicles from the other cells of the vascular compartment were examined to select a suitable panel of antibodies to analyse these same vesicle types in plasma samples from non-pregnant (NonP), normal pregnant (NormP) and PE women. Our flow cytometer was shown to detect microvesicles ≥290nm, hence smaller nanovesicles and exosomes could not be detected by this method. Therefore, a novel technique for analysing both microvesicles and nanovesicles, Nanoparticle Tracking Analysis (NTA), was explored and was found to be able to detect vesicles as small as 70nm. The origins of the vesicles that change in pregnancy are not yet known. Flow cytometry and NTA were used in parallel to determine the size, number and phenotype of STBM and other cellular vesicles in NonP, NormP and PE women. Flow cytometry showed that majority of vesicles were derived from platelets, followed by RBC vesicles, leukocyte vesicles and STBM. NTA showed that the total number of vesicles in plasma was significantly elevated in NormP and late-onset PE women compared to NonP controls, and the vesicles were smaller in size. Similarly, flow cytometry showed differences in the composition of vesicles between pregnant and non-pregnant women, demonstrating that pregnancy affects vesicle release. However, no differences were found between NormP and PE women. This was probably due to the majority of samples studied being from late rather than early-onset PE. Thus, although this is the most comprehensive analysis of circulating vesicles in pregnancy to date, their use as biomarkers for PE remains an open question.