Fetal anaemia

Definition

• Fetal anaemia is a reduction of fetal haemoglobin below the value expected for gestational age.

Aetiopathogenesis

Causes

• The leading cause is severe fetal anaemia.
• Classic triggers are parvovirus B19, fetomaternal haemorrhage.
• A pseudo-sinusoidal pattern follows opioids such as pethidine and is shorter, less regular and interrupted by accelerations.

Aetiology

• The most important immune cause is alloimmunisation: a Rh-negative mother forms antibodies after exposure to Rh-positive fetal red cells, which in subsequent pregnancies cross the placenta and destroy fetal red cells.
• Antibodies against other blood group systems such as Kell, Duffy, Kidd or Cc and Ee can also cause haemolytic disease; anti-Kell antibodies additionally suppress erythropoiesis in the bone marrow.
• Parvovirus B19 infects fetal tissues expressing the P antigen receptor, including haematopoietic cells; fetuses in the first and second trimesters are particularly at risk because red cell progenitors develop mainly during this period.

Clinical features

Clinical features and complications

• Fetal anaemia can lead to hypoalbuminaemia, high-output heart failure, hydrops fetalis and fetal death; ultrasound signs of hydrops are ascites, skin oedema and pleural and pericardial effusions.
• After birth, in haemolytic causes, jaundice and hyperbilirubinaemia up to kernicterus predominate.

Diagnosis

Second role of the MCA: Anaemia diagnostics

• Regardless of the PI, the peak systolic velocity (PSV) in the MCA is the crucial marker for fetal anaemia.
• A value of of ≥ 1.5 MoM, indicating cordocentesis suggests moderate to severe anaemia.
• Reasons for anaemia diagnostics include, for example, Rhesus incompatibility, a parvovirus B19 infection or TAPS in the anemic donor twin.

What it looks like

• The sinusoidal pattern is a smooth, sinusoidal line without variability.
• Amplitude and frequency are 5–15 bpm and 3–5 cycles/minute.
• The criterion is a duration of at least 30 minutes; accelerations are completely.
• Under FIGO the sinusoidal pattern counts straight away as pathological.

Typical imaging findings

• CTG: The trace runs in even, smooth waves around 130–140 beats, with a regular frequency of about three to five cycles per minute and constant amplitude. Short-term variability and accelerations are completely absent; the pattern persists over many minutes.
• CTG: Around a baseline of about 150 per minute runs an even, wave-like oscillation without any irregularity. Accelerations are absent and the tocogram shows only irregular small deflections.

• Non-invasively, the peak systolic velocity of the middle cerebral artery is measured; from 18 weeks a value above 1.5 MoM strongly suggests moderate to severe fetal anaemia.
• In alloimmunisation, antibody identification, the father's antigen status and serial titres are used for assessment; fetal haemoglobin is determined by umbilical cord blood sampling.

Keep learning in the app

Further reading (selection)

1. StatPearls: Hemolytic Disease of the Fetus and Newborn (NCBI Bookshelf)
2. MSD Manual Professional Edition: Hemolytic Disease of the Fetus and Neonate
3. J Pers Med 2024: Parvovirus B19 Infection and Pregnancy – Review of the Current Knowledge (PMC, open access)
4. DocCheck Flexikon, Arteria cerebri media
5. DocCheck Flexikon, Kardiotokographie

Cross-references