Evaluate and manage thrombocytopenia and haematological disorders in pregnancy

In one line

Thrombocytopenia complicates 7–12% of pregnancies; the task is not to count platelets but to separate benign gestational thrombocytopenia from the dangerous mimics — immune thrombocytopenia (ITP), pre-eclampsia/HELLP, and the thrombotic microangiopathies (TTP, atypical HUS, acute fatty liver) — because a wrong label drives the wrong delivery decision.

This chapter assumes the platelet-physiology and pre-eclampsia groundwork in hypertension in pregnancy and pre-eclampsia & HELLP basics; the consultant-level content is the discriminators, the subtype-specific regimens, and the judgement calls. The question is not what a falling platelet count is but what it means when the smear, the gestation and the renal function point in different directions.

Assessment

The diagnosis is a pattern-recognition and exclusion problem, not a count. Four mechanistic buckets generate almost every case, and each has a signature read off the bloods before the label is written.

1. Dilution / accelerated clearance (gestational thrombocytopenia, GT). Plasma-volume expansion plus mildly increased platelet turnover at the placental bed. 75–80% of all cases; a third-trimester, asymptomatic, >80 × 10⁹/L phenomenon that resolves by 1–2 months postpartum. It has no first-trimester form and never causes neonatal thrombocytopenia.

2. Immune destruction (ITP, HIV-related, drug-induced, SLE/APS). Antiplatelet (or, in HIV, immune-complex and molecular-mimicry) clearance by the reticuloendothelial system. The mechanism that matters: the antibody crosses the placenta, so unlike GT it can — uncommonly — drop the neonatal count. ITP is the commonest cause of a count <50 × 10⁹/L before 20 weeks.

3. Consumption (pre-eclampsia/HELLP, DIC, abruption, sepsis, TTP/aHUS). Platelets are consumed in a microangiopathic or coagulopathic process. The signature is that thrombocytopenia travels with other organ injury — haemolysis, transaminitis, renal failure, fibrinogen consumption — not in isolation.

4. Underproduction (marrow failure, B12/folate deficiency, advanced HIV, infiltration). Rare but the one bucket a single blood film and reticulocyte count will catch — look for a leuco-erythroblastic film, macrocytosis, or pancytopenia.

The discriminators that decide the diagnosis:

• Timing and trajectory. A count <100 × 10⁹/L in the first or early second trimester, or a steadily falling count, points away from GT and toward ITP or an evolving consumptive process. GT has no early-pregnancy phenotype; if you see one, it is not GT.
• The number itself, read against the gestation. Below 70 × 10⁹/L GT becomes statistically improbable — in dedicated series the GT nadir rarely falls below this. Below 50 × 10⁹/L actively hunt an alternative diagnosis. Isolated thrombocytopenia ≠ benign.
• The smear is the single most informative cheap test. Schistocytes flag a microangiopathy (TTP/HUS/HELLP); platelet clumping flags EDTA-induced pseudo-thrombocytopenia (repeat in citrate or by a manual count before you commit a patient to a workup); a leuco-erythroblastic picture flags marrow infiltration; hypersegmented neutrophils + macrocytosis flag B12/folate. Add LDH, haptoglobin, reticulocytes, creatinine, transaminases, urate, a coagulation profile and fibrinogen.
• The SA-specific filter. In an antenatal population with HIV seroprevalence above a quarter, HIV is a leading remediable cause — via immune destruction (which can occur at a preserved CD4 count and may be the presenting feature) and, in advanced disease, marrow suppression. Test and stage HIV before labelling any thrombocytopenia "gestational" or "immune", and remember that ART-related and TB-treatment–related cytopenias add a drug axis.
• No diagnostic test confirms ITP or GT — both are diagnoses of exclusion. Bone marrow and antiplatelet-antibody assays are not required to diagnose ITP in pregnancy, and a normal marrow does not exclude it.

The atypical and the deceptive

The straightforward picture is the asymptomatic third-trimester woman with a count of 110. The harder ones do not read cleanly:

• The GT/ITP overlap zone (50–80 × 10⁹/L, third trimester, no prior counts). This is the genuinely hard call, and it is usually unresolvable in real time. When pre- and post-pregnancy counts are later checked, roughly one in three women labelled GT actually meet ITP criteria — so the safest consultant move in the overlap zone is to manage the count, document the uncertainty, and book a postpartum recheck at 6–8 weeks, which retrospectively makes the diagnosis (normalised → GT; persistently low → ITP). An experimental discriminator exists — serum thrombopoietin runs far higher in ITP than GT — but it is not an NHLS routine and does not change acute management.
• Severe isolated thrombocytopenia (<20–30 × 10⁹/L) with a normal smear and normal organs. This is ITP until proven otherwise; GT essentially never does this, and a consumptive process would derange something else.
• Thrombocytopenia that worsens after delivery. HELLP and acute fatty liver improve once the placenta is out; a count that keeps falling on day 2–3 postpartum, especially with rising creatinine or LDH, is TTP or atypical HUS revealing itself — the placenta was never the driver.
• "HELLP" that will not resolve. Persistent haemolysis and thrombocytopenia beyond ~72 h postpartum should reopen the microangiopathy differential rather than be accepted as slow HELLP recovery.

Severity stratification and the high-stakes split

Stratify by count band (driving bleeding and procedural risk) crossed with organ involvement (driving the diagnosis and delivery decision). A count of 60 with normal organs and a stable trend is a different patient from a count of 60 with rising LDH and creatinine, even though the number is identical.

The single highest-stakes split is HELLP vs TTP vs aHUS vs acute fatty liver (AFLP), because management diverges sharply:

HELLP and AFLP improve with delivery; TTP and aHUS do not — delivering a TTP patient instead of starting plasma exchange is a classic, fatal error. Three mechanism→consequence links carry this table: (1) TTP is an ADAMTS13 problem, so the deficiency is severe (≤10%) and the haemolysis is the most florid — LDH is highest and schistocytes most numerous; (2) aHUS is a complement-regulation problem centred on the renal endothelium, so renal failure dominates and is disproportionate to the BP and liver; (3) AFLP is a hepatic mitochondrial/fatty-acid-oxidation problem, so it uniquely couples thrombocytopenia with hypoglycaemia and true coagulopathy (low fibrinogen, prolonged PT) — the combination that should never be called "HELLP".