Large studies have shown 12% of pregnant woman can have a platelet count less than 150×109/L by late pregnancy.1 Therefore, most experts recommend the lower limit of normal for platelets in pregnancy to be classed as 100-115 ×109/L.2,3
Thrombocytopenia between 100-150 ×109/L will not usually affect the woman’s pregnancy. However, as a rule of thumb platelet counts of <80 ×109/L will have an impact on the ability to have an epidural and platelets <50 ×109/L will have implications for caesarean section and spontaneous vaginal delivery.4
Common causes (not an exhaustive list):
- Gestational thrombocytopenia
- Usually occurs in the third trimester and causes mild thrombocytopenia i.e platelets 100-150×109/L, but they can go as low as 70 ×109/L3. This is caused by normal physiological dilutional effects and increased platelet destruction and accounts for approximately 75% of thrombocytopenia in pregnancy.
- The platelet count should improve to normal by 12 weeks post-partum.
- Platelet counts of <70 ×109/L should prompt further investigation as gestational thrombocytopenia doesn’t typically cause platelets <70 ×109/L. A platelet count of <100 ×109/L may also merit referral to allow preparation for delivery/neuraxial anaesthesia.
- Acute viral infections
- Check full medication history. Consider heparin induced thrombocytopenia (HIT) if the patient is on treatment dose low molecular weight heparin in pregnancy with new onset thrombocytopenia within 5-10 days.
- Falsely low count caused by platelet clumping in EDTA or presence of large platelet causing analyser to underestimate platelet count- best ruled out by a blood film and then repeat platelet count in citrate (blue coag tube).
- B12 and/or folate deficiency
- Immune thrombocytopenia
- Caused by platelet destruction due to platelet auto-antibodies. Thrombocytopenia can vary from mild to very severe (e.g <5 ×109/L). Patients with ITP require monitoring during pregnancy and after delivery, and may require treatment - there is a higher risk of maternal bleeding when the platelet count is <50 ×109/L at delivery. There is a risk of thrombocytopenia in the newborn which is importantly not proportional to maternal platelet count e.g Mother’s platelet count could be 70 ×109/L and foetus’s could be <5 ×109/L and vice versa.
- ITP accounts for approximately 5% of cases of thrombocytopenia in pregnancy and is a diagnosis of exclusion.
- Antiphospholipid syndrome and SLE
- Disseminated intravascular coagulation
- In pregnancy most often occurs in sepsis or haemorrhage.
- Thrombotic thrombocytopenic purpura
- There is an increased risk of TTP in pregnancy. It is a life-threatening condition with low platelets, haemolytic anaemia, renal impairment, neurological symptoms and fever but not all need to be present for a diagnosis. TTP can present at any time in the pregnancy. Needs urgent obstetric and haematology review.
- Pre-eclampsia is a disease occurring in the second half of pregnancy (after 20 weeks gestation), and conventionally characterised by pregnancy-induced hypertension, with proteinuria and often oedema. Needs urgent obstetric review.
- HELLP (haemolysis, elevated liver enzymes and low platelets) syndrome
- This usually presents in women who have pre-eclampsia or eclampsia, therefore occurs in the last half of the pregnancy. 70% of cases present before delivery, peaking between 27 and 37 weeks of gestation but it can occur earlier or later. 30% of women with HELLP syndrome present postpartum, usually within 48 hours of delivery. Requires urgent obstetric review.
- Rare causes;
- Acute fatty liver- tends to present in late pregnancy with nausea, vomiting and abdominal pain, fevers, headache and jaundice. Needs urgent obstetric review.
- Hereditary thrombocytopenia accounts for <1% thrombocytopenia in pregnancy.
- Type 2b Von Willebrand disease.
- Bone marrow disorders- Women can still present with leukaemia or lymphoma in pregnancy.
- Is the patient well?
- Any recent viral infections?
- Change to medication?
- Patient history of previous thrombocytopenia inside/outside pregnancy?
- Family history of bleeding problems both inside or outside of pregnancy?
- Does the patient have a personal history of bleeding problems outside of pregnancy?
- Gestation at onset can gives clues to aetiology (not an exhaustive list)
- Anytime; ITP, TTP.
- Second and third trimesters; Gestational thrombocytopenia.
- After 20 weeks; Pre-eclampsia, HELLP.
- After 35 weeks; Fatty liver of pregnancy/HELLP/Pre-eclampsia.
- Any bruises or petechia? More likely to be relevant if large bruises or raised from skin.
- Any lymphadenopathy?
- Any features of underlying connective tissue disease?
- Repeat FBC if first instance of low platelets to confirm.
- Blood film- (please add clinical details including gestation to aid film interpretation) for B12/folate deficiency/giant platelets/platelet clumping/red cell fragmentation.
- Send a citrate (blue coag tube) for FBC if there is platelet clumping on the film, or a flow cytometry sample if large/giant platelets are seen for an “immuno-platelet count”.
- U+E for renal impairment- TTP/HUS.
- LFTs- check for high bilirubin in haemolysis (TTP, HUS) or deranged LFTs in HELLP/acute fatty liver.
- Coagulation screen (DIC).
- Autoantibody screen +- Double stranded DNA.
- Blood pressure and urine dipstick for protein.
- HIV, Hep B and Hep C testing.
If the patient is otherwise well, with no evidence of other possible aetiologies and the other FBC parameters and blood film, U+E and LFTs and baseline observations are all normal then a patient with platelets >100 ×109/L will need to be informed their platelet count is low and it will need to be monitored during the pregnancy at least every 4 weeks.
Platelet counts of <100 ×109/L only occur in approx. 1% of pregnant woman so referral for specialist investigation in secondary care is then appropriate (check local guidance for referral threshold).
If the patient is unwell with thrombocytopenia or is bleeding in association with thrombocytopenia then they require urgent assessment.
An FBC should be performed at 12 weeks post-partum to ensure the thrombocytopenia has resolved as gestational thrombocytopenia should have resolved by this time- consider onwards referral if the thrombocytopenia is still persistent and the cause unknown.
Boehlen F, Hohfeld P, Extermann P et al. Platelet count at term pregnancy: a reappraisal of the threshold. Obstetrics and Gynecology 2000; 95:29–33.
Pavord, S., & Hunt, B. (Eds.). (2010). The Obstetric Hematology Manual. Cambridge: Cambridge University Press. p5
ASH Quick reference 2013 Clinical practice guide on thrombocytopenia in pregnancy. A Rajasekhar et al.
Estcourt, L. J., Birchall, J. , Allard, S. , Bassey, S. J., Hersey, P. , Kerr, J. P., Mumford, A. D., Stanworth, S. J., Tinegate, H. and , (2017), Guidelines for the use of platelet transfusions. Br J Haematol, 176: p377 doi:10.1111/bjh.14423