During pregnancy, care providers typically will seek to monitor iron levels. They do this by checking hemoglobin — usually with a fingerstick, which is a quick and simple way to measure levels in the office without lab work. These measurements are then used as the basis for a number of clinical decisions, such as whether to put you on iron supplements (not something to be taken lightly, since excess iron is toxic) or even whether to restrict what kind of care provider you can use or where you have to give birth. This approach to iron in pregnancy has two major problems:
- Hemoglobin is not iron.
- “Normal” ranges are not created with pregnant women in mind.
Hemoglobin is Not Iron
The first issue is that hemoglobin is not iron. It is used as a proxy for iron because that fingerstick is a simpler and more accessible way to measure levels on a routine basis than drawing blood and doing labs repeatedly.
But it isn’t actually iron. Hemoglobin is an element of the blood that requires iron to build it. Iron deficiency is one of the most common reasons — if not the most common reason — for hemoglobin to be low. But it isn’t the only reason.
Iron isn’t the only thing necessary for the body to build hemoglobin, and if any one of those other things is lacking — such as the B vitamins necessary for the body to make good use of its iron stores — the result can be low hemoglobin.
What all of this means is that those fingersticks are not a bad way to routinely check on iron. But if the hemoglobin level comes up low, that should not be a basis for determining care. It should be a basis for ordering labwork to get a better idea of what is really going on. This is true whether the person being tested is pregnant or not.
“Normal” is Not Normal in Pregnancy
During pregnancy, there’s yet another factor at play, though: the “normal” range was not determined with pregnant women in mind. Most of the time, it’s simply assumed that pregnant women should have levels that fall into the same range that’s “normal” for everyone else. This overlooks some very basic truths about physiology (function of the body) during pregnancy.
See, when we measure hemoglobin, it isn’t an absolute number; it’s essentially a percentage. If that’s hard to get your head around, try this: think of a pitcher of water with ice cubes in it. You could count the ice cubes in the pitcher and say that it has “10 ice cubes.” Or you could measure relative to the water, and say that you have “10 ice cubes per half gallon.” When we measure hemoglobin, it’s a measurement more like that second one.
Why is this important? Because one of the most important changes to the body during pregnancy is the expansion of the blood volume.12 The body needs more blood, so it can pump through all of mom’s blood vessels and the placenta. And it seems to undergo some other changes in the process, like thinning out some in order to circulate freely through the placenta. It would seem, from the research that has been done on pregnant women (see below), that hemoglobin is not designed to increase along with the more “watery” portions of the blood.
Going back to our pitcher of water, the pitcher is now fuller. The number of ice cubes hasn’t increased or decreased. There are exactly the same number of ice cubes as before. We still have “10 ice cubes.” But now, if we measure them relative to the water, we have “10 ice cubes per three-quarters of a gallon.”
Is “Low” Good or Bad?
We have a problem here, though, because there is a disconnect between the research and most practitioners, and because they aren’t considering the underlying physiology, they’re overlooking something critical.
They’re looking for numbers that are “normal” for people who aren’t pregnant. Unfortunately, while numbers like that might mean you gained hemoglobin (added a few more ice cubes to the pitcher), it’s very likely that they mean the blood volume is not expanding like it needs to be (the pitcher isn’t much fuller than when it started). This is a serious problem, with the potential to lead to premature delivery, eclampsia, etc.3
So a wise practitioner should know that a “good” hemoglobin level should actually be a red flag to check for hydration and adequate intake of salt, protein, and calories in general.4
What “Low” Looks Like in Pregnancy
So far I’ve given you some explanations and I’ve made some claims, but I haven’t shown you much evidence, so let’s take a look at that. There is relatively little research on hemoglobin during pregnancy, but what research there is consistently shows better outcomes at levels that are slightly lower than what is typically considered “normal.” Levels that are considered “high normal” show pretty bad outcomes.
This graph (source) of outcomes in a study about preterm labor demonstrates a similar pattern to most of the others:
Studies found that “high levels of hemoglobin, hematocrit, and ferritin are associated with an increased risk of fetal growth restriction, preterm delivery, and preeclampsia.” “…iron supplements and increased iron stores have recently been linked to maternal complications (eg, gestational diabetes) and increased oxidative stress during pregnancy.” “Failure of hemoglobin to fall below 105 g/L* was associated with increased risk of poor outcome…”5
“Haemoglobin concentrations <95 g/l** seem to be remarkably harmless.” 6
“There is evidence of the higher risk of stillbirth in women with high hemoglobin level (146 g/l*** and above)…” 7
“…infant GM [gross motor] scores increased sharply with increasing maternal Hb concentration until 90 g/L where increasing GM was mild, and began to decline after 110 g/L.” Translation: Gross motor skills among the newborn babies were best when their mothers’ hemoglobin had been between about 9 and 11. They were poorer when maternal hemoglobin levels were lower than 9 or higher than 11. 8
*In the U.S., this is usually referred to as 10.5, because we record the measurement based on a different overall volume.
**9.5 according to most U.S. readings
***14.6
The CDC is Aware
There’s no good explanation for this disconnect between research and practice. The CDC is aware. They acknowledged in 1989 that “current major reference criteria for anemia…are not based on representative samples and fail to take into account the normal hematologic changes occurring during pregnancy.” 9 So they made some small adjustments. But they didn’t make their adjustments based on naturally healthy pregnant women. They made their adjustments based on women whose iron levels were being artificially raised! 10
Communicate with Your Practitioners
As I’ve shown, aiming for hemoglobin levels a bit lower than currently considered “normal” is solidly evidence-based. It is possible to still be iron-deficient during pregnancy, but it’s more difficult to identify this through the normal testing routes and, as a general rule, it is probably better to focus on good blood volume expansion and whether symptoms of iron deficiency are present.
If all other indicators are good, and hemoglobin is “low,” the low hemoglobin is probably a sign of good blood expansion, and nothing to worry about. If hemoglobin is “good,” and other things are “off,” it’s probably important to follow up on blood volume expansion.
But convincing practitioners is the hard part. If you need some help with this, you can download the following document, which I created to give to my own practitioners. It’s designed to briefly summarize the research, for medical practitioners who understand and will care about the science, but who may not want to spend the time to read all the studies.
- https://cebp.aacrjournals.org/content/16/9/1720.short
- https://www.sciencedirect.com/science/article/abs/pii/0002937883908566
- https://www.physiology.org/doi/abs/10.1152/jappl.1993.75.4.1566
- https://academic.oup.com/jn/article-abstract/109/11/1887/4770606
- http://ajcn.nutrition.org/content/81/5/1218S.long
- http://www.bmj.com/content/310/6978/489?ijkey=acd66cbfddeb03e3aa3bfb98c3cb0b9e2f831dbc&keytype2=tf_ipsecsha
- https://www.ncbi.nlm.nih.gov/pubmed/26639719
- https://www.ncbi.nlm.nih.gov/pubmed/26055847
- https://wonder.cdc.gov/wonder/prevguid/p0000169/p0000169.asp
- https://www.cdc.gov/mmwr/preview/mmwrhtml/00051880.htm
