How to Read Your Iron Blood Test in Pregnancy - Haemoglobin, Ferritin, Transferrin and CRP Explained

You've had your bloods done. The results are back a table of markers, numbers and reference ranges that may or may not include a note from your GP.

Most women are only told about haemoglobin. But haemoglobin is a late-stage marker. By the time it drops out of the normal range, iron stores have often been depleted for months. Treating pregnancy iron status based on haemoglobin alone means deficiency is caught late, managed reactively, and often inadequately.

This guide covers every marker on a complete iron panel - what it measures, what's considered normal, what's optimal for pregnancy, and the critical context (CRP) that changes how you interpret everything else.

The complete iron panel - all six markers explained

A full iron study should include all of the following. Here's what each one tells you:

Marker	Normal Range	Concerning If...	What It Means
Haemoglobin	115-165 g/L (pregnancy)	< 110 g/L (T1/T3), < 105 (T2)	Measures red blood cells. Low = anaemia but a late sign. Stores already depleted by this point.
Ferritin	> 30 µg/L (ideally > 50)	< 15 µg/L = depleted	Your iron storage tank. The most sensitive early marker. Test this every trimester.
Serum Iron	10-30 µmol/L	< 10 µmol/L	Iron circulating in the blood right now. Fluctuates with recent meals - less reliable alone.
Transferrin	2.0-3.6 g/L	Elevated in deficiency	The protein that carries iron through the blood. Rises when iron is low - your body making more 'trucks' for limited cargo.
Transferrin Saturation	20-50%	< 15% = deficiency	% of transferrin carrying iron. Low % = plenty of trucks, not enough cargo. A key functional iron marker.
CRP	< 5 mg/L	Elevated = inflammation	Not an iron marker but critical context. Elevated CRP artificially raises ferritin. Always check CRP alongside iron studies.

† Reference ranges are a general guide only and may vary between pathology laboratories. Individual results should always be interpreted in the context of your symptoms, health history and in consultation health professional.

Haemoglobin - what it tells you and what it misses

Haemoglobin (Hb) measures the concentration of haemoglobin protein in your red blood cells - the protein that carries oxygen from your lungs to every cell in your body.

Haemoglobin thresholds for anaemia in pregnancy:

• First trimester: below 110 g/L
• Second trimester: below 105 g/L (haemodilution lowers Hb physiologically - this is normal)
• Third trimester: below 110 g/L

The critical limitation: Your body maintains haemoglobin as a priority, drawing on ferritin stores to do so. This means ferritin can be severely depleted with all the associated symptoms of iron deficiency while haemoglobin remains within the normal range.

Iron deficiency without anaemia (IDWA) is extremely common in pregnancy and is associated with fatigue, cognitive impairment and mood disturbance even in the absence of anaemia. It will not be captured by haemoglobin testing alone.

Ferritin - the most important marker most women never get tested

Ferritin is the protein that stores iron within cells. Serum ferritin is the most sensitive and reliable indicator of total body iron stores and the marker most likely to reveal early deficiency before haemoglobin is affected.

Reference ranges vs optimal ranges

The laboratory reference range for ferritin is typically listed as 15-200 µg/L. The lower bound of 15 µg/L is the threshold for iron deficiency not the threshold for adequate stores in pregnancy.

• Optimal ferritin for pregnancy: Ideally above 50 µg/L.
• Minimum adequate: Above 30 µg/L. Ferritin between 15-30 µg/L is technically 'normal' but represents minimal reserve - insufficient buffer for the escalating iron demands of the second and third trimester.

How frequently to test ferritin

Ferritin should be tested each trimester, not just at booking. Iron demands increase significantly from the second trimester as fetal growth accelerates and a ferritin that was adequate at 10 weeks may be depleted by 24 weeks without supplementation.

Postpartum ferritin testing at 6 weeks is also important birth involves significant blood loss and further depletes stores that were already under pressure.

Serum iron - what it adds

Serum iron measures the amount of iron circulating in the blood at the time of the test. Unlike ferritin, which reflects longer-term stores, serum iron fluctuates with recent dietary intake and the time of day.

This variability makes serum iron less reliable as a standalone marker but it adds useful context when interpreted alongside ferritin and transferrin saturation. Low serum iron alongside low ferritin and low transferrin saturation provides a more complete picture of functional iron deficiency.

Transferrin and transferrin saturation - the transport system

Transferrin: Transferrin is the protein that transports iron through the bloodstream - think of it as the truck that carries iron cargo to where it's needed. When iron stores are low, the liver produces more transferrin in an attempt to capture and deliver whatever iron is available.

An elevated transferrin alongside low ferritin and low serum iron is a classic pattern of iron deficiency - the body making more trucks for dwindling cargo.

Transferrin saturation: Transferrin saturation (TSAT) measures what percentage of available transferrin is actually carrying iron (how full the trucks are).

• Normal range: 20-50%
• Below 15%: indicates functional iron deficiency - transferrin is circulating with very little iron attached
• In pregnancy: TSAT naturally decreases somewhat due to increased transferrin production - values should be interpreted in context

Transferrin saturation is particularly useful because it reflects whether there's enough iron available for functional use - red blood cell production, enzyme function, oxygen transport independent of what ferritin says about storage.

CRP - the marker that changes everything else

CRP (C-reactive protein) is a marker of systemic inflammation. It is not an iron marker but it is essential context for interpreting ferritin accurately.

Ferritin is an acute phase reactant. In response to inflammation - whether from infection, illness, autoimmune activity, or physiological stress the liver releases ferritin into the bloodstream independently of iron stores.

The clinical implication: Elevated CRP causes ferritin to appear falsely elevated. A ferritin of 60 µg/L alongside a CRP of 30 mg/L does not mean iron stores are adequate, it means inflammation is masking the true picture

Why this matters in late pregnancy: Late pregnancy involves a degree of physiological inflammation - CRP may be mildly elevated even in healthy women.

If your ferritin looks 'fine' in the third trimester but you feel exhausted and symptomatic, check CRP alongside it. Always request CRP at the same time as your iron studies. Without it, ferritin cannot be accurately interpreted in isolation.

A CRP above 5 mg/L warrants careful interpretation of ferritin results.

What to request at your next appointment

These are the specific questions and requests to bring to your antenatal appointment:

• 'Can I have a full iron study - ferritin, serum iron, transferrin and transferrin saturation, not just haemoglobin?'
• 'Can you also run CRP at the same time so we can interpret the ferritin in context?'
• 'What is my ferritin number specifically? Has it changed since my last test?'
• 'Is my ferritin above 50 µg/L or just within the normal reference range?'
• 'If my ferritin is below 30 µg/L, what are our options for supplementation?'

You are entitled to know your numbers, not just whether they've been flagged. A ferritin of 16 µg/L and a ferritin of 55 µg/L will both come back 'normal' on a standard lab report. Only by knowing the actual value can you make informed decisions about supplementation and diet.

Frequently asked questions

1. Why does my ferritin look normal but I still feel exhausted? Two possible explanations: First, your ferritin may be in the lower end of the normal range (15-30 µg/L) technically normal, but inadequate for pregnancy demands. Second, if CRP is elevated, your ferritin may be artificially inflated by inflammation. Always check CRP alongside ferritin, and ask for the actual number rather than just a 'normal' or 'abnormal' flag.

2. How often should I get my iron tested in pregnancy? At a minimum: first trimester (baseline), second trimester (24-28 weeks, when iron demands increase significantly), and third trimester (32-36 weeks, ahead of birth). Women with known iron deficiency, heavy periods, PCOS, or a previous pregnancy with iron deficiency may benefit from more frequent monitoring.

3. What does it mean if my transferrin is high? Elevated transferrin in the context of low ferritin and low transferrin saturation is a classic pattern of iron deficiency. Your liver is producing more of the transport protein in response to low iron availability. This pattern - high transferrin, low TSAT, low ferritin is a clear signal for iron supplementation.

Related reading

→ Part 1: Why Women Enter Pregnancy Iron Depleted 

→ Part 2: The Problem With Ferrous Sulphate 

→ The Prenatal - 19 nutrients for pregnancy