Full Blood Count in Pregnancy: What It Shows
What a full blood count measures
A full blood count (FBC), also called a complete blood count, is one of the most common blood tests in UK pregnancy. It measures the three main components of your blood:
- Red blood cells - which carry oxygen around your body
- White blood cells - which fight infection
- Platelets - which help your blood clot
Each component tells your midwife and GP something different about your health. An FBC is usually done at your booking appointment (8-12 weeks) and again around 28 weeks.
What the results show
Haemoglobin (Hb)
Haemoglobin is the protein in red cells that carries oxygen. Low Hb indicates anaemia, which in pregnancy is most often caused by iron deficiency. Normal ranges are slightly lower in pregnancy because your blood volume expands by up to 50%, diluting the concentration of red cells.
Mean corpuscular volume (MCV)
MCV measures the average size of your red cells. A low MCV suggests iron-deficiency anaemia. A high MCV may point to a deficiency in vitamin B12 or folate, which is also checked in some pregnancies.
White blood cell count
White cells rise naturally during pregnancy, particularly in the third trimester and labour. A very high count with symptoms may indicate infection. A very low count is less common and may need investigation.
Platelets
Platelets help your blood clot. Pregnancy naturally lowers platelet count slightly. If it drops too far, you may bruise easily or bleed more than expected. Very low platelets (thrombocytopenia) may need specialist review and can affect options for epidural or spinal anaesthesia.
Why the FBC is repeated
Your midwife usually arranges an FBC at booking and again at around 28 weeks. This is because:
- Anaemia often develops later in pregnancy as your baby's iron demands increase
- Platelets can fall as pregnancy progresses
- It gives a baseline before labour in case you need a transfusion
Interpreting results in pregnancy
Pregnancy changes normal blood values. For example:
- A haemoglobin below 110g/L in early pregnancy, or below 105g/L later, is typically classified as anaemia in the UK
- Platelets below 150 x10^9/L may need review
- White cells up to 13 x10^9/L can be normal in late pregnancy
Always discuss results with your midwife rather than comparing them to non-pregnancy reference ranges.
What happens if results are abnormal
Low haemoglobin (anaemia)
Treatment depends on severity:
- Mild: iron-rich diet advice and over-the-counter iron
- Moderate: prescribed iron tablets (often ferrous sulphate)
- Severe: iron infusion or, rarely, blood transfusion
Side effects of iron tablets include constipation and dark stools. Taking iron with vitamin C (such as orange juice) can improve absorption.
Low platelets
Mildly low platelets may just be monitored. If significantly low, your obstetrician will investigate causes such as gestational thrombocytopenia, immune thrombocytopenia, or pre-eclampsia.
High white cells
This usually prompts a check for infection, including urine tests and swabs.
Preparing for the test
The FBC does not usually require fasting. Tell your midwife about any medications, especially iron supplements or blood-thinning drugs.
A simple test with big insights
The FBC is quick, cheap and very informative. It catches anaemia, infection and clotting issues early - all of which are easier to treat in pregnancy than in emergency situations. Make sure you attend your booking and 28-week blood tests, and ask for your results if they are not mentioned.