Rh Negative Pregnancy Notes

Rh Antigens

There are five Rh antigens: small c, capital C, capital D, capital E, and small e. These antigens are proteins found on the surface of red blood cells and are genetically determined.

These antigens are located on the short arm of chromosome one, specifically the RHD and RHCE genes. The RHD gene encodes the D antigen, while the RHCE gene encodes the C, c, E, and e antigens.

Among these, the capital D antigen is the most significant. Its presence indicates that the individual is Rh positive. Approximately 85% of the Caucasian population is Rh positive. Conversely, its absence means the individual is Rh negative.

These Rh antigens appear as early as 38 days of gestation on the red blood cells (RBCs). This early expression is crucial in the context of Rh incompatibility, as it can affect the fetus during pregnancy.

Immune Response in Rh Negative Individuals

If an Rh negative individual is exposed to Rh positive RBCs, their immune system recognizes the Rh antigen as foreign. This exposure can occur through blood transfusion or during pregnancy when fetal RBCs enter the maternal circulation.

This recognition triggers the production of antibodies against the Rh antigen by the Rh negative individual's immune system. This process is known as alloimmunization, where the body mounts an immune response against foreign antigens from the same species.

Rh Negative Pregnancy Defined

Rh negative pregnancy is defined as a situation where the mother is Rh negative and the fetus is Rh positive. This condition can lead to Rh incompatibility, posing significant risks to the fetus.

If both mother and fetus are Rh negative, it is not considered a high-risk pregnancy because there is no Rh antigen mismatch to trigger an immune response.

For a pregnancy to be classified as high-risk Rh negative, the mother must be Rh negative, and the fetus must be Rh positive. This is the classical scenario for Rh incompatibility.

The scenario where the mother is Rh positive and the fetus is Rh negative does not constitute an Rh negative pregnancy related risk, as the mother's immune system will not react against the fetus's RBCs.

The possibility of the mother being Rh negative and the fetus being Rh positive arises when the father is Rh positive. If the father is heterozygous for the D antigen, there is a chance the fetus will inherit the Rh positive gene.

In such cases, there is a 50% chance of the fetus being Rh positive and a 50% chance of being Rh negative. This probability depends on the genetic makeup of the father.

If both parents are Rh negative, all fetuses will be Rh negative, which does not present a high-risk scenario because there is no Rh antigen present to cause maternal sensitization.

Initial Antenatal Visit

During the first antenatal visit, ABO Rh typing is performed to determine the patient's Rh status. This is a standard part of prenatal screening to identify potential risks associated with blood group incompatibilities.

If the patient is identified as Rh negative, the next step is to ascertain the husband's Rh status. This is crucial for assessing the risk of Rh incompatibility.

If the husband is also Rh negative, the pregnancy is considered low risk because the fetus will also be Rh negative. However, if the husband is Rh positive, the pregnancy is classified as high risk due to the potential for the fetus to be Rh positive.

Risks Associated with Rh Negative Pregnancy

The primary concern in Rh negative pregnancies is the potential for fetal blood to leak into the mother's blood during pregnancy, a phenomenon known as fetal maternal hemorrhage. This can occur spontaneously or during invasive procedures.

When fetal maternal hemorrhage occurs, and the fetus is Rh positive while the mother is Rh negative, the Rh antigens on the fetal RBCs can trigger the mother's immune system. The volume of fetal blood that enters the maternal circulation influences the degree of sensitization.

The mother's immune system recognizes the Rh antigens as foreign, leading to the production of antibodies against them. This process is called Rh alloimmunization.

Initially, the antibodies produced are IgM antibodies, which cannot cross the placenta, thus posing no immediate threat to the fetus. This is why the first pregnancy is usually safe in Rh negative mothers.

IgG antibodies, which can cross the placenta, are formed later in the pregnancy. These antibodies are smaller and can pass through the placental barrier, posing a risk to subsequent Rh positive fetuses.

Subsequent Pregnancies

In subsequent pregnancies, if the fetus is Rh positive and fetal maternal hemorrhage occurs, the mother's immune system responds more rapidly due to prior sensitization. This is because memory B cells are already primed to produce antibodies against the Rh antigen.

Both IgM and IgG antibodies are produced quickly. The IgG antibodies can cross the placenta if produced in significant amounts, leading to potential complications for the fetus.

Once in the fetal circulation, these IgG antibodies target the Rh antigens on the fetal RBCs, leading to hemolysis. This antibody-mediated destruction of fetal RBCs is the primary mechanism behind Rh disease.

Consequences of Hemolysis in the Fetus

Hemolysis in the fetus leads to fetal anemia. The severity of anemia depends on the amount of IgG antibodies present and the rate of RBC destruction. Severe fetal anemia can result in heart failure, a condition known as hydrops fetalis.

Hydrops fetalis is diagnosed via ultrasound and is not a clinical diagnosis. It is characterized by fluid accumulation in at least two fetal compartments, such as ascites, pleural effusion, pericardial effusion, and skin edema.

Fetal anemia can be detected through:

  • Sinusoidal heart wave pattern on CTG or NST. This pattern indicates severe fetal anemia and hypoxia.

  • Increased peak systolic velocity in the middle cerebral artery (more than or equal to 1.5 MoM). This is a non-invasive method to assess fetal anemia, as blood viscosity decreases with anemia, leading to increased blood flow velocity in the MCA.

Other consequences of hemolysis include jaundice, hepatosplenomegaly, and bone marrow hyperplasia (erythroblastosis fetalis). Jaundice results from the accumulation of bilirubin, while hepatosplenomegaly and bone marrow hyperplasia are compensatory mechanisms to produce more RBCs.

Placentomegaly may also occur as the placenta attempts to compensate for reduced nutrient and oxygen supply to the fetus. The placenta enlarges to increase its functional capacity.

Rh negative pregnancy primarily poses a risk to the fetus, potentially leading to fetal morbidity and mortality. Without proper management, severe Rh incompatibility can result in stillbirth or significant neonatal complications.

Maternal Complications

Rh negative pregnancies can lead to maternal complications such as:

  • Placentomegaly: Enlargement of the placenta due to increased workload and compensatory mechanisms.

  • Pregnancy-induced hypertension (PIH): Also known as pre-eclampsia, characterized by high blood pressure and proteinuria. It may be related to placental dysfunction.

  • Polyhydramnios, due to increased maternal transudation across the placenta. The exact mechanism is not fully understood but may involve increased fetal urine production or altered amniotic fluid dynamics.

Prevention of Fetal Complications

To prevent fetal complications, anti-D immunoglobulin is administered to the mother. Anti-D is a purified IgG antibody against the RhD antigen.

Anti-D acts as a "policeman" in the mother's circulation, destroying fetal RBCs before they can stimulate the mother's immune system. This prevents the mother from developing her own anti-D antibodies.

Anti-D is given in a low concentration 300
micro g, which is sufficient to neutralize 30 mL of fetal maternal hemorrhage or 15 mL of fetal RBCs. The standard dose effectively covers most FMH volumes.

This low concentration ensures that even if anti-D crosses the placenta, it will cause limited hemolysis, preventing severe fetal complications. The amount of hemolysis caused by anti-D is minimal and does not pose a significant risk to the fetus.

Anti-D is only effective if given before the fetal RBCs stimulate the mother's immune system, i.e., in Rh negative unsensitized mothers. Once the mother has developed her own anti-D antibodies, anti-D immunoglobulin is no longer effective.

Indirect Coombs Test

The indirect Coombs test is used to determine whether a mother is sensitized. It detects the presence of anti-Rh antibodies in the mother's serum.

A negative result indicates an unsensitized female, while a positive result indicates a sensitized female. A positive result means the mother has already developed anti-Rh antibodies.

In Rh negative females, the indirect Coombs test is performed at the first antenatal visit and repeated at 28 weeks of pregnancy. This screening helps identify women who require anti-D prophylaxis.

If the test remains negative, anti-D is administered at 28 weeks. This is a routine antepartum prophylaxis to prevent Rh sensitization.

Management of Unsensitized Rh Negative Females

For unsensitized Rh negative females (negative indirect Coombs test):

  • Administer anti-D at 28 weeks (antepartum prophylaxis). This reduces the risk of sensitization from any FMH that may have occurred.

  • Deliver between 39 to 40 weeks via vaginal delivery. There is no specific indication for cesarean section based solely on Rh negative status.

  • Perform early cord clamping to prevent sensitization during delivery. Early cord clamping reduces the risk of fetal blood entering the maternal circulation.

  • Check the Rh status of the newborn. This is essential to determine if the newborn is Rh positive and if postpartum anti-D is needed.

  • If the newborn is Rh positive, perform a direct Coombs test on the newborn's blood. This test detects if the newborn's RBCs are coated with maternal antibodies.

  • If the direct Coombs test is negative, administer anti-D to the mother (postpartum prophylaxis) within 72 hours of delivery, but it can be given up to 28 days. Postpartum anti-D prevents sensitization from any FMH that occurred during delivery.

Postpartum sterilization does not preclude the administration of anti-D. Even if the woman will not have future pregnancies, anti-D is still given to prevent sensitization from the current pregnancy.

Management of Sensitized Rh Negative Females

If the indirect Coombs test is positive (sensitized female), anti-D is not beneficial. In this case, the mother has already developed anti-Rh antibodies, and anti-D will not reverse this.

In such cases, the next step is to determine the antibody titer to assess the amount of anti-D antibodies present. The antibody titer helps determine the severity of Rh alloimmunization.

The critical titer is 1:16. Antibody titers are serially diluted in mother's blood, and the presence of antibody is checked. 1:2, 1:4, 1:8 dilutions are considered insignificant numbers of antibodies. If a 1:16 dilution demonstrates the presence of antibodies, these are considered significant. This titer is a threshold above which fetal monitoring is initiated.

If the antibody titer is less than 1:16, repeat the titer every four weeks. If the titer starts increasing, repeat it every two weeks. Monitoring the titer helps track the progression of alloimmunization.

Begin fetal monitoring at 32 weeks and deliver between 37 to 38 weeks. Fetal monitoring includes NST and biophysical profiles to assess fetal well-being.

If the antibody titer is more than or equal to 1:16, perform Doppler of the middle cerebral artery to check the peak systolic velocity. This assesses for fetal anemia.

  • If peak systolic velocity is less than 1.5 MoM, repeat it every one to two weeks, perform fetal surveillance weekly (NST and biophysical profile), and deliver between 37 to 38 weeks. This indicates mild to no anemia.

  • If peak systolic velocity is more than or equal to 1.5 MoM, perform cordocentesis to measure fetal hemoglobin and hematocrit. This confirms the presence and severity of fetal anemia.

Management Based on Cordocentesis Results

  • If fetal hemoglobin is less than two standard deviations below normal or hematocrit is more than 30% (mild anemia), repeat hemoglobin and hematocrit weekly, perform fetal surveillance weekly, and deliver between 37 to 38 weeks. Mild anemia can be managed with close monitoring.

  • If fetal hemoglobin is more than two standard deviations below normal or hematocrit is less than 30% (severe anemia):

    • If gestational age is more than or equal to 35 weeks, deliver. Delivery is preferred to reduce further risks of anemia.

    • If gestational age is less than 35 weeks, perform intrauterine blood transfusion. This corrects the anemia and allows the fetus to continue to develop in utero.

In sensitized Rh negative pregnant females, vaginal delivery is preferred, and delayed cord clamping is recommended. Vaginal delivery is safe as long as fetal monitoring is reassuring. Delayed cord clamping allows for maximal blood volume transfer to the fetus, especially important in cases of anemia.

Fetal Maternal Hemorrhage Assessment

After delivery, fetal maternal hemorrhage is typically less than 4 mL. However, certain conditions can increase fetal maternal hemorrhage:

  • Abdominal trauma

  • Instrumental delivery

  • Twin deliveries

  • Intrauterine death of the fetus

  • Manual removal of placenta

To assess fetal maternal hemorrhage, perform a rosette test. If positive, proceed with a Kleihauer-Betke test to quantify the volume of fetal maternal hemorrhage.

The Kleihauer-Betke test should be done within two hours of delivery and helps calculate the required dose of anti-D. The test quantifies the number of fetal RBCs in maternal circulation.

For every 1 mL of extra fetal blood, administer an additional 10 \mu g of anti-D. This ensures adequate neutralization of fetal RBCs.

Singer's alkali denaturation test can differentiate vasa previa from placenta previa but is only qualitative. Kliehauer-Betke is quantitative and should be performed instead or as well.

Indications and Dosage of Anti-D

First Trimester:

  • After abortion (spontaneous or induced)

  • After ectopic pregnancy

  • After molar pregnancy

  • After chorionic villi sampling

Dosage: 50 \mu g if pregnancy is less than 12 weeks (as per ACOG guidelines).

Second and Third Trimesters:

  • After amniocentesis

  • After external cephalic version or internal cephalic version

  • After any unexplained bleeding

  • After antepartum hemorrhage or fetal death

  • After abdominal trauma

  • At 28 weeks of pregnancy for all Rh negative females with a negative indirect Coombs test

  • After delivery, if the fetus is Rh positive and the direct Coombs test is negative

Dosage: 300 \mu g intramuscularly.

Case Studies on Anti-D Administration

Cordocentesis:

  • Do NOT give anti-D after cordocentesis because the only criteria for cordocentesis is mom is Rh negative, the indirect Coombs test is positive, and peak systolic velocity is > 1.5 MOM.

Anti-D Administration During Pregnancy:

  • If a pregnant female has received anti-D during pregnancy and delivers within three weeks, do NOT administer anti-D after delivery because the half-life of anti-D is 28 days.

Minor Antigens

Minor antigens with a risk of sensitization:

  • Kell

  • Kidd

  • Duffy A

Minor antigens without a risk of sensitization:

  • Lewy

  • I

  • Duffy B

Kell antigen most commonly leads to sensitization after blood transfusion. Anti-Kell antibodies can cause severe fetal anemia due to suppression of erythropoiesis.

Anti-E alloimmunization is the most common non-D alloimmunization.

Anti-C alloimmunization is the most severe non-D alloimmunization. It can lead to significant fetal morbidity and mortality.

Mirror Syndrome (Ballantyne Syndrome)

Mirror syndrome occurs when both the mother and fetus appear as mirror images of each other. It is characterized by fetal hydrops and maternal edema, hypertension, and proteinuria.

For example, in an Rh negative sensitized mother with severe fetal hemolysis, the mother may exhibit polyhydramnios and pregnancy-induced hypertension, leading to a bloated appearance. The baby may be born with hydrops, pericardial effusion, and ascites, also appearing swollen.

Mirror syndrome can also be seen in twin-to-twin transfusion syndrome, cystic hygromas, and placental chorioangiomas. It is associated with severe fetal morbidity and mortality.

Hydrops Fetalis
Definition:

Hydrops fetalis is an ultrasound-based diagnosis characterized by the presence of any two of the following four findings:

  • Pericardial effusion

  • Ascites

  • Pleural effusion

  • Subcutaneous edema

Polyhydramnios and placentomegaly may be present, but are not included in the diagnostic criteria. These findings are supportive but not diagnostic.

First Sign:

The first sign of hydrops fetalis on ultrasound is scalp edema, appearing as a halo around the fetal head (Buddha sign). This is often the earliest detectable sign.

Types:

Immune hydrops fetalis: Occurs due to Rh negative pregnancy.

Nonimmune hydrops fetalis: Occurs due to other causes. Nonimmune hydrops fetalis is more common than immune hydrops fetalis.

Causes of Nonimmune Hydrops Fetalis:

  • Cardiovascular anomalies (most common)

  • Chromosomal anomalies

  • Infections (e.g., parvovirus B19, TORCH infections)

  • Anemias and thalassemias

Suspect hydrops fetalis in Rh negative pregnancies if fetal hemoglobin is less than 5 g/dL or hematocrit is less than 15%. These values indicate severe anemia and warrant further investigation.