Pregnancy is a major physiological stress of the cardiovascular system. Weight gain significantly contributes to physical limitations. This thesis examines the effects on both physical and cardiac performance of weight gain in pregnancy. Utilising cardiac power output at rest and maximal exercise, I measured the effects of (i) inert weight loading (ii) pregnancy and (iii) obesity in the non-pregnant state, to determine the acute, chronic and also reversible changes. Weight loading using a pregnancy simulator suit (“Empathy Belly”) showed reduced physical performance, whilst showing an improvement in cardiac performance, predominantly by increasing the pressure generating capacity of the heart. Additional load carriage with the “Empathy Belly” and a rucksack, showed further reduction in physical performance, but no further improvement in cardiac performance. Pregnancy revealed significant reductions in physical performance and maintenance in cardiac performance compared to the non-pregnant post-partum. Contrary to this, there were significant reductions in both physical and cardiac performance in pregnancy, compared to pre-conception. Changes in cardiac performance throughout pregnancy gradually improved, whilst there was a deterioration in overall physical performance. Obesity in the non-pregnant state, showed significant reduction in physical performance with a marked increase in cardiac performance. This was primarily driven by an increase in the flow generating capacity of the heart; the cardiac output. Inert weight loading, weight carriage in pregnancy and non-physiological weight gain in obesity in the non-pregnant state, all reduce physical performance. In contrast to this, both inert weight carriage and weight carriage in obesity increase cardiac performance. Acute weight loading induces an increase in pressure generating capacity, whilst chronic weight carriage leads to an increase in flow generating capacity. For the first time, I have shown that peak cardiac performance reduces in pregnancy from pre-conception, although this gradually improves throughout pregnancy and is likely to be in part caused by an increase in weight gain.