Genetics and conflict probability
The main cause of isoserological incompatibility is the difference in antigens of the Rh system. Conflict occurs only if the mother has Rh-negative blood (genotype dd) and the father is Rh-positive.
The probability of the fetus inheriting the Rh factor directly depends on the father's genotype:
- If the man is homozygous (DD) (which accounts for 40–45% of all Rh-positive men), the dominant gene is always transmitted. The fetus will be Rh-positive in 100% of cases.
- If the man is heterozygous (Dd) (55–60% of cases), the probability of having an Rh-positive child is 50%.
Note: In immunohematology, the symbol "d" is used conditionally. It denotes not an independent gene (its existence has not been confirmed), but the physical absence of the D antigen on erythrocytes.
How maternal sensitization occurs
To trigger an immune response, fetal erythrocytes must enter the maternal bloodstream. This fetomaternal transfusion most frequently occurs during labor (especially during cesarean section or manual placenta removal).
Other risk factors disrupting the integrity of chorionic villi:
- Abortions (induced and spontaneous) and ectopic pregnancy.
- Gestational complications: preeclampsia, premature placental abruption, threatened miscarriage.
- Invasive procedures: amniocentesis, chorionic villus sampling, cordocentesis.
- Iatrogenic immunization (transfusion of Rh-positive blood to a woman in the past).
The immune response develops in two stages. During the initial contact, complete antibodies — IgM — are produced. They have a large molecular weight and do not cross the placental barrier. Upon re-exposure to the antigen (secondary response), the body begins massive synthesis of incomplete antibodies — IgG. These easily pass through the placenta and attack the fetus.
Pathogenesis of hemolytic disease of the fetus (HDF)
Maternal IgG antibodies undergo the antigen-antibody reaction with fetal erythrocytes, triggering their destruction — hemolysis. This leads to severe anemia and the accumulation of toxic unconjugated bilirubin (jaundice).
Trying to compensate for anemia, the fetus increases erythropoietin synthesis. When the bone marrow can no longer cope, extramedullary hematopoiesis is triggered in the liver, spleen, kidneys, adrenal glands, and placenta.
Cascade of HDF complications:
- The enlarged liver compresses the umbilical and portal veins $\rightarrow$ portal hypertension develops.
- The protein-synthesizing function of the liver is impaired $\rightarrow$ hypoproteinemia develops.
- Colloid-osmotic pressure drops $\rightarrow$ fluid leaks into tissues, forming ascites and generalized edema.
- The heart operates in a hyperdynamic mode (increased cardiac output) $\rightarrow$ myocardial hypertrophy is replaced by heart failure against the background of tissue hypoxia and acidosis.
In addition, unconjugated bilirubin is lipophilic. It penetrates brain tissue and damages its nuclei, which manifests in the neonatal period as bilirubin encephalopathy (kernicterus).
Diagnostic Principles
Diagnostics are conducted in two directions: confirming maternal sensitization and assessing the severity of the fetal condition.
In Rh-negative women, antibody screening begins early — at 6–12 weeks of gestation, with mandatory titer monitoring over time. Special attention is paid to patients with an burdened history (blood transfusions without considering Rh factor, antenatal fetal death, or birth of children with HDN in the past).
To diagnose the condition of the fetus, the following methods are used:
- Ultrasound fetometry and placentometry.
- Middle cerebral artery Doppler velocimetry (allows detection of anemia).
- Investigation of amniotic fluid and fetal blood.
- General assessment of the fetal condition.
Briefly: answers to topical questions
Which specific ultrasound markers indicate the development of the hydrops form of hemolytic disease of the fetus?
The development of the hydrops form of hemolytic disease of the fetus is indicated by specific ultrasound markers, including changes in the placenta, internal organs, and fluid accumulation.
- Pronounced placentomegaly — an increase in placental thickness up to 6.0–8.0 cm.
- Hepatosplenomegaly — enlargement of the liver and spleen.
- Ascites — accumulation of fluid in the abdominal cavity.
- Polyhydramnios — excessive amount of amniotic fluid.
- Cardiomegaly — enlargement of the heart.
- Pericardial effusion — fluid in the pericardial cavity.
- Hydrothorax — fluid in the pleural cavity.
- Increased echogenicity of the intestine — occurs as a result of edema of its wall.
- Subcutaneous edema — localized in the area of the head, torso, and limbs of the fetus.
Which specific Doppler parameter is used for the non-invasive diagnosis of severe fetal anemia?
For the non-invasive diagnosis of fetal anemia, the peak systolic velocity of blood flow in the fetal middle cerebral artery is used.
- Parameter: peak velocity of blood flow / peak systolic velocity in the middle cerebral artery.
- Pathological criterion: increase in peak blood flow velocity; a value of ≥ 1.5 MoM corresponds to zone A and indicates moderate or severe anemia.
There is an inverse correlation between the peak blood flow velocity in the middle cerebral artery and the fetal hemoglobin level: velocity increases as hemoglobin decreases.
What are the indications for cordocentesis and intrauterine fetal blood transfusion?
Indications for cordocentesis and intrauterine blood transfusion depend on the results of non-invasive diagnostics and the severity of fetal anemia.
- Indications for cordocentesis: bilirubin optical density level in zone 3 or 2C on the Liley scale, presence of ultrasound markers of hemolytic disease, high peak systolic velocity in the middle cerebral artery (≥ 1.5 MoM), as well as suspected hypoxia, anemia, or intrauterine infection.
- Indications for intrauterine blood transfusion: a decrease in hematocrit by 15% or more from the norm and/or a hemoglobin deficit of more than 70 g/L for the given gestational age.
At what critical antibody titer value in maternal blood is an advanced invasive fetal examination indicated?
Advanced invasive fetal examination is indicated when the critical titer of anti-Rh antibodies in the maternal blood reaches 1:16, 1:32, or higher. When such a titer is detected at 26–28 weeks of gestation, the pregnant woman undergoes transabdominal amniocentesis. The purpose of this study is to assess the concentration of bilirubin in amniotic fluid, which is necessary to decide on the advisability of intrauterine blood transfusion. At the same time, the titer itself is not an absolutely accurate criterion for disease severity, and its instability is considered the most unfavorable sign.
To avoid confusing immunoglobulins, use the association: IgM — Massive (too large, do not cross the placenta), IgG — Flexible (easily penetrate to the fetus and cause hemolysis).
Class and exam questions
Can a conflict arise if the mother has blood group 1 (O) and the fetus has group 2 (A)?
Yes, this is ABO incompatibility. The mechanism is similar: fetal antigens trigger the production of immune α- and β-antibodies in the mother. However, this form of conflict is significantly milder than Rh immunization and usually does not require intensive care.
Why is the absolute value of the maternal antibody titer not of decisive importance?
The titer indicates only the degree of the mother's immune response. A sensitized woman with a high titer may give birth to an absolutely healthy Rh-negative child (if the father was heterozygous and transmitted the "d" gene).
What other blood systems can cause immunological conflict during pregnancy?
In addition to the Rh factor and the ABO system, hemolytic disease can be triggered by antigens of the Kell-Cellano, Duffy, Kidd, MNSs, and Lutheran systems.
What else to study on this topic
- Ultrasound markers of generalized fetal hydrops in hemolytic disease of the fetus
- Middle cerebral artery Doppler: how fetal anemia is detected
- Invasive methods: indications for amniocentesis and cordocentesis
- Mechanisms of bilirubin encephalopathy and kernicterus
- ABO incompatibility: pathogenesis and clinical features
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