I have a sneaking suspicion that the extra blood volume during pregnancy dilutes the effect of my daily SSRI, but have assumed that no one has researched this. Is there actually any data to support this?
—Doing My Best
This is an interesting theory! You’re right that there is no direct evidence, although we can think about the logic behind your theory.
Your blood volume increases during pregnancy. How much varies by person, but it’s usually about 40% higher. This higher blood volume helps support the hard work of building a baby. Interestingly, there is a larger proportional increase in plasma than in red blood cells. An explanation for this is that there is a lot of growth in organs like your skin, which need blood but not too much oxygen (so they are relatively less greedy for red blood cells). Regardless of the reason: blood volume goes up and then comes back down after birth.
However, it does not seem likely that this impacts your antidepressant. I say this for the simple reason that SSRI dosing isn’t weight-based, or blood-volume-based. People typically start with the same dose, and then dosing is adjusted up or down based on response. Dosing isn’t changed when people gain or lose weight (which would also impact blood volume). All this together suggests that it’s unlikely the blood volume changes have affected your response.
Having said this: it is entirely possible that other changes in your hormones have impacted your response to your SSRIs during pregnancy. There is not much data on how people respond differentially to these drugs while pregnant, so even putting aside the question of blood volume, we just don’t know if this is an expected reaction.
Whatever the cause, if your SSRIs are not dosed right, you should talk to your doctor! Even if we have no idea why, it is worth fixing.
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