Fetal Thrombocytopenia

Fetal thrombocytopenia is a fetal platelet count below 150 x 10^9/L, and severe disease is below 50 x 10^9/L. The commonest severe cause is fetal and neonatal alloimmune thrombocytopenia, in which maternal antibodies (usually anti-HPA-1a) destroy fetal platelets, and other causes are maternal immune thrombocytopenia, infection, chromosomal abnormality and severe growth restriction. It matters because intracranial haemorrhage can occur from 20 weeks and cause death or disability.

• Alloimmune disease often affects the first pregnancy.
• Maternal immune thrombocytopenia less often causes severe fetal thrombocytopenia.
• A previous affected sibling predicts severity.

Women are well, with a normal platelet count in alloimmune disease or a low count in maternal immune thrombocytopenia. Findings come from scans (intracranial haemorrhage, ventriculomegaly) or a previous affected baby.

• Previous baby with bruising, bleeding, brain haemorrhage or low platelets.
• Fetal ventriculomegaly or porencephaly on scan.
• Differential diagnosis: maternal immune thrombocytopenia, infection (cytomegalovirus, toxoplasma, rubella, HIV), trisomy 13, 18 or 21, triploidy, severe growth restriction, Rhesus disease, drug effect.
• Complications: intracranial haemorrhage, fetal death, neonatal bleeding, neurological disability.

The approach is to find the cause by maternal and paternal tests and to confirm fetal platelet count where needed.

• Maternal full blood count and platelet count (usually normal in alloimmune disease), HPA genotyping of both parents, maternal anti-HPA antibodies, antinuclear antibody, HIV and hepatitis tests, TORCH serology.
• Fetal platelet count by cordocentesis or by postnatal count of the previous baby when available; avoid fetal sampling if platelet count is expected to be very low unless compatible platelets are available.
• Ultrasound and MRI of the fetal brain for haemorrhage, ventriculomegaly, porencephaly.
• Amniocentesis for karyotype and cytomegalovirus PCR when anomalies or growth restriction are found.

The principle is to treat the mother with immunoglobulin and steroids in alloimmune disease, to protect from birth injury and to give compatible platelets at birth.

• Supportive: refer to a fetal medicine unit; avoid trauma and unnecessary fetal sampling; avoid NSAIDs in the mother; plan delivery in a unit with a blood bank and neonatal intensive care.
• Pharmacological: alloimmune disease: immunoglobulin 1 g/kg IV weekly from 20 weeks (from 12 to 16 weeks if a previous baby had an intracranial haemorrhage) plus prednisolone 0.5 mg/kg oral daily (maximum 40 mg) with calcium carbonate 1 g oral daily; maternal immune thrombocytopenia: prednisolone 0.5 to 1 mg/kg oral daily (maximum 60 mg) or immunoglobulin 1 g/kg IV daily for 1 to 2 days when platelets are below 30 x 10^9/L before delivery; betamethasone 12 mg IM twice, 24 hours apart, between 24 and 34 weeks and 6 days if preterm delivery is planned; neonatal immunoglobulin 1 g/kg IV and, for platelets below 30 x 10^9/L or below 50 x 10^9/L with bleeding, compatible platelets 10 to 15 mL/kg IV.
• Procedures: intrauterine platelet transfusion by fetal medicine for severe disease; caesarean section at 36 to 37 weeks for severe alloimmune disease or fetal platelets below 50 x 10^9/L; avoid vacuum, forceps, scalp electrodes and fetal scalp sampling; cranial ultrasound of the newborn within 24 hours.
• Special groups: maternal immune thrombocytopenia has maternal platelets checked every 2 to 4 weeks and weekly from 36 weeks, and the newborn has a platelet count at birth and on days 2 to 5; infection related thrombocytopenia is managed by treating the infection.
• Red flags: fetal ventriculomegaly or brain haemorrhage on scan, maternal platelets below 30 x 10^9/L, fetal platelets below 20 x 10^9/L, reduced fetal movements, bleeding.
• Follow-up: scan every 2 to 4 weeks; newborn platelet count at birth, daily until above 50 x 10^9/L and rising, then at 1 week; neurodevelopmental review; counselling on recurrence risk, which is very high when the father is homozygous.