Preeclampsia is a hypertensive disorder of pregnancy affecting the maternal cardiovascular system. Relaxin, a secretory product of the corpus luteum (CL), is a potent vasodilator facilitating haemodynamic adaptations to pregnancy and normal implantation. However, whether there is an association between serum relaxin and cardiovascular health in pregnancy remains uncertain. We investigated the association between serum relaxin and cardiovascular parameters in pregnancy using longitudinal data and serum samples obtained through the Oxfordshire Women and Their Children's Health (OxWATCH) study. Serum relaxin and plasma placenta-derived angiogenic factors were measured using ELISA in the first and third trimesters of pregnancy. Among 47 participants, there was a 51 % reduction in relaxin between the first and third trimesters (P = 0.003), and significant increases in all measured angiogenic factors, including soluble Fms-like tyrosine kinase-1 (sFlt-1), soluble endoglin (SENG) and placental growth factor (PlGF). Serum relaxin did not differ between stratified blood pressure (BP) groups. 'Lower normal BP' participants (n = 29) displayed a greater decline (57 %) in relaxin levels in the third trimester than in 'higher normal BP' women (38 %), although this did not reach statistical significance. 'Higher normal BP' participants (n = 18) displayed higher SENG at 7 weeks (P = 0.033). Serum relaxin correlated negatively with PlGF (r = -0.69; P = 0.016). In conclusion, we found no differences in serum relaxin between groups; however, differences in relaxin's decline into the third trimester indicate a potential mechanistic shift in the regulation of its release from the CL in later pregnancy, warranting further research.