Just guessing, but maybe due to fluid retention in both cases leading to "dilution" of the 'erythrocyte sediments' so the rate at which things sediment is faster?
Although, looking up ESR on Wiki gives that its increased in preg and decreased in CHF.
However, digging into this a little more:
http://www.ncbi.nlm.nih.gov/pubmed/11762656
- UK study showing that ESR increased with pregnancy
http://www.nejm.org/doi/full/10.1056/NEJM199102073240601
- NEJM paper showing that ESR is of limited value in CHF
http://forums.studentdoctor.net/threads/a-clearly-basic-question-on-esr.984304/
- SDN thread, 2nd post down:
Erythrocyte Sedimentation Rate (ESR)
Mechanism Tested:
Products of Inflammation (ex.
Fibrinogen) coat RBCs and cause Red Blood Cell Aggregation making them fall faster in test tube
Conditions with Effects on ESR:
Elevated ESR -
Infection
Inflammation (ex. Temporal Arteritis)
Cancer
Pregnancy (increased fibrinogen)
SLE
Polymyalgia Rheumatica
Low ESR -
Sickle Cell Disease (changed shape)
Polycythemia (too many)
CHF (unknown)