Low Platelets in Pregnancy

Why you were told at all

You did not ask for a platelet count. It arrived because it is part of a full blood count, and NICE guideline NG201 says to offer a blood test to check full blood count, blood group and rhesus D status at the first antenatal appointment, and a further full blood count with blood group and antibodies at 28 weeks.

ACOG's Practice Bulletin No. 207 on thrombocytopenia in pregnancy makes exactly this point in its opening line: obstetricians "frequently diagnose thrombocytopenia in pregnant women because platelet counts are included with automated complete blood cell counts obtained during routine prenatal screening". It is found because it is looked for, not because anything prompted the search.

Two ways of writing the same number

ACOG's bulletin flags a source of confusion worth clearing up before anything else. US practice reports platelets in Conventional Units and most other countries use SI units, and the conversion factor between them is 1.0. So 150,000 per microlitre and 150 x 10⁹ per litre are the same count written two ways. If you are comparing a UK result with an American page, the digits look wildly different and the value is identical.

The three explanations, and how they are told apart

Gestational thrombocytopenia

This is by far the most common cause of a low platelet count in pregnancy. Platelet counts fall modestly across normal pregnancy through dilution and increased turnover. It appears in the second half of pregnancy, is mild, causes no symptoms, does not affect the baby, and resolves after birth. There is no treatment because none is needed.

Immune thrombocytopenia

Immune thrombocytopenia, or ITP, is an autoimmune destruction of platelets. It is much less common than gestational thrombocytopenia but matters more. Features that point towards it are a low count in the first trimester, before gestational thrombocytopenia typically appears, a count that is substantially lower than the mild reductions of pregnancy, a history of easy bruising or bleeding, or a previously documented low count outside pregnancy. Unlike gestational thrombocytopenia, the antibodies can cross the placenta, so the baby's platelet count is checked after birth.

Pre-eclampsia and HELLP

This is the diagnosis that changes management most urgently. NICE guideline NG133 lists "fall in platelet count (under 150,000/microlitre)" among the features, alongside a rise in alanine transaminase over 70 IU/litre or twice the upper limit of normal, that indicate severe disease and inform decisions about birth. NG133 also names "progressive deterioration in liver function, renal function, haemolysis, or platelet count" among the criteria for escalation.

What separates this from the other two is that it does not come alone. A falling platelet count with rising blood pressure, protein in the urine, abnormal liver enzymes or upper abdominal pain is a different situation from an isolated mildly low count in a well woman with a normal blood pressure.

What your team will actually do

The investigation is usually simple: repeat the count to confirm it, check a blood film, check liver and kidney function, check blood pressure and urine, and take a history of bleeding and of previous counts. Where an autoimmune cause is suspected, testing may extend to lupus and antiphospholipid antibodies, which is why the NHS pages on lupus and antiphospholipid syndrome are relevant here even though most people with low platelets have neither.

The single most useful piece of information is a platelet count from before pregnancy, if one exists. It converts an ambiguous result into a clear trend. Check whether you have had a blood test in the past few years and tell your midwife.

The epidural question

This is what most people are really asking, and the honest answer is that there is no single universal cut-off. Anaesthetic practice uses a threshold below which the risk of spinal haematoma is considered to outweigh the benefit of regional anaesthesia, and that threshold varies between units and is applied alongside the trend, the cause and whether you are on any anticoagulant.

The practical consequence is that if your count is low, you should be referred for an antenatal anaesthetic review rather than left to find out in labour. NICE guideline NG121 asks for intrapartum care to be planned in advance with a multidisciplinary team where an existing medical condition is present, and this is precisely that situation. Ask for the referral; do not wait to be offered it.

A repeat count close to term is usually arranged for the same reason, because a count taken at 28 weeks does not tell the anaesthetist what your count is at 39 weeks.

Birth and afterwards

Mild thrombocytopenia does not, on its own, change how you give birth. Where the count is significantly low or the cause is ITP, the plan may include treatment to raise the count before birth, avoiding certain instrumental procedures and fetal blood sampling, and checking the baby's platelet count after delivery. Treatments for ITP in pregnancy include corticosteroids and immunoglobulin; they are named here without doses because they are specialist prescribing decisions.

After birth, NICE guideline NG133 says that in women who had pre-eclampsia with mild or moderate hypertension, platelet count, transaminases and serum creatinine should be measured 48 to 72 hours after birth, and not repeated if those results are normal. Gestational thrombocytopenia resolves in the weeks after birth, so a repeat count at your postnatal check confirms it and closes the question.

When to seek help

A low platelet count itself rarely causes symptoms at the levels usually found in pregnancy. What matters is bleeding that is out of proportion: spontaneous bruising, bleeding gums, nosebleeds that will not stop, or blood in urine or stool. Those are reasons to be seen. So is a headache with visual disturbance or upper abdominal pain, which points at pre-eclampsia rather than at the platelets themselves.