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I'm pregnant. I've learnt that during my pregnancy my blood volume will increase by 50%. https://www.ncbi.nlm.nih.gov/m/pubmed/4075604/

So I'm wondering, what does my body do to decrease my blood volume after I give birth? Where does all that extra blood go? I would imagine I lose some during labour, but I seriously doubt it's all of it.

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The extra volume of fluid is due to the fluid retaining effects of high oestrogen secretion in a pregnant human female. After this high level of oestrogen subsides the extra fluid is mainly eliminated as:

  • urination

  • some is lost during parturition (labour)

  • lactation to some extent as well

Reference:A Textbook of Medical Physiology (South Asian Edition) Guyton and Hall

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    $\begingroup$ Note that this process can take some time, and in the meantime you may see some of that extra fluid show up as edema. My ankles stayed trim throughout my entire twin pregnancy, but swelled up to twice their normal width within a day of delivery. Also, you'll continue to bleed (like a heavyish period) for a couple of weeks after delivery. $\endgroup$ – 1006a Nov 7 '17 at 4:23
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The actual blood cells just get circulated out over time (e.g. broken down in the spleen). Eliminating the fluid is the easy part - consider how easy it is to drink a liter of water, absorb it and add to your plasma volume, and then filter it straight back out through the kidneys in a matter of an hour or so.

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    $\begingroup$ The other answer elaborates on how the fluid leaves the body, but completely glosses over what happens to the blood cells. I suggest elaborating on how the fluid leaves the body. $\endgroup$ – wizzwizz4 Nov 5 '17 at 17:07
  • $\begingroup$ You phrased that very nicely. $\endgroup$ – RedSonja Nov 6 '17 at 9:29

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