Maternal Red Blood Cell Alloimmunization Research Articles

Maternal red blood cell alloimmunization prevalence in the United States

AbstractHemolytic disease of fetus and newborn (HDFN) is a life-threatening disease mediated by maternal alloimmunization to red blood cell (RBC) antigens. Studies of maternal alloimmunization prevalence in the United States lack national data. This study describes prevalence and trends in alloimmunization in pregnancy in the United States. RBC antibodies (abs) were identified in a large, nationwide, commercial laboratory database from 2010 to 2021. The cohort comprised pregnancies for which the year of laboratory collection and patient’s state of residence were available. Data were normalized based on US Centers for Disease Control and Prevention estimates of live births and weighted by year and US Census Division. Cochrane-Armitage tests assessed temporal trends of alloimmunization. Of 9 876 196 pregnancies, 147 262 (1.5%) screened positive for RBC abs, corresponding to an estimated prevalence of 1518 of 100 000 pregnancies. Of identified RBC abs, anti-D comprised 64.1% pregnancies (586/100 000). Prevalence of other high-risk RBC abs for HDFN included anti-K (68/100 000) and anti-c (29/100 000). Incidence of all 3 high-risk abs increased from 2010 to 2021 (all P < .001). Among almost 10 million pregnancies in the United States, comprising an estimated 14.4% of all pregnancies, 1.5% screened positive for RBC abs. Almost three-quarters (683/100 000 [74.3%]) of RBC abs identified were high risk for HDFN. Although prevalence of anti-D is difficult to interpret without the ability to distinguish alloimmunization from passive immunity, it remains problematic in HDFN, ranking second only to anti-K in critical titers. Given the sequelae of HDFN, new initiatives are required to reduce the incidence of alloimmunization in patients of reproductive potential.

Maternal Red Blood Cell Alloimmunization Managed with Intrauterine Blood Transfusion: Predictors of Poor Outcome

Background: The rhesus (Rh) and Kell blood group systems are the most common of over 50 different antigens capable of causing maternal red blood cell (RBC) alloimmunization and severe fetal hemolytic disease. Anti-K and anti-D are responsible for a significant proportion of fetal anemia requiring intrauterine transfusion (IUT). Whilst IUT of packed RBCs improves neonatal survival and morbidity, clinical prognostic indicators are lacking. Our primary objective was to identify predictors of adverse outcome. Methods: We conducted a retrospective single-center study at Mount Sinai Hospital (MSH), Toronto, Canada. All pregnant patients alloimmunized with anti-K and anti-D as a single antibody, between 1991 and 2018 were included. Data were obtained from patient medical records, ultrasound reports and information from the transfusion medicine laboratory. Data included maternal demographics, antibody titers, pregnancy history, number of IUTs, hemoglobin (HB) concentration at the beginning and end of all IUTs. Neonatal outcomes included survival, mode of delivery, gestational age at delivery, birth weight, HB at birth and need for neonatal transfusion, phototherapy or intravenous immunoglobulins (IVIG). Our primary outcome was the composite outcome of stillbirth or neonatal death (SB/NND). We also constructed a secondary outcome consisting of top-up neonatal transfusion, exchange transfusion, phototherapy, or use of IVIG. Medians and interquartile ranges (IQR) or mean±SD were used as summary statistics and compared by Mann-Whitney or t-test; p&amp;lt;0.05 was statistically significant. Outcome predictors were identified using multivariable logistic or linear regression analysis; accounting for patient level clustering by marginal Generalized Estimating Equation did not significantly affect variables. Data were analyzed using SPSS. Results: 116 women with 128 pregnancies and 425 IUTs with anti-K or anti-D as a single antibody were identified. Median maternal age was 31 years (27.0-35.0) for anti-K and 32 years (23.6-40.6) for anti-D. The gestational age at 1st IUT differed significantly between anti-K and anti-D (24.3 vs 28.7 weeks respectively, p=0.004). Women with anti-K antibodies required more IUTs than women with anti-D (3.84 vs 3.12 IUTs, p=0.036) and HB at 1st IUT was significantly lower in the anti-K group (5.10 vs 7.05 g/dL, p=0.001) (Table 1). Following initiation of IUT, the time from 1st IUT to delivery was 69.6 days in the anti-K group and 54.6 in the anti-D group (p=0.06). The daily decrease of HB between 1st and 2nd IUT (as a marker of disease severity), development of fetal hydrops and severe preterm birth did not differ significantly between the two groups. Mean gestation age at delivery was 35.0 weeks in the anti-K and 36.0 weeks in the anti-D group (p=0.28), with 87.1% and 93.9% survival (p=0.37), respectively. The proportion of neonates requiring phototherapy, IVIG and exchange/top-up transfusion was comparable across the two antibody groups (Table 1). Regression analysis showed that delivery occurred sooner if HB dropped more rapidly between the first two IUTs (p=0.01). Each additional transfusion gained on average 22.5 days in utero (Table 2). In multivariable analysis, gestational age at 1st IUT was the only predictor of a SB/NND outcome (adjusted OR 0.79 [95%CI 0.67-0.93]; p=0.006). With 1st IUT at 23 weeks, the risk of SB/NND was 8%, but only 2.5% at 28 weeks and &amp;lt;0.01% at 36 weeks (Table 3). Gestational age at delivery (adjusted OR 1.11 [95%CI 1.00 to 1.23]; p=0.046) and HB at 1st IUT (adjusted OR 0.84 [95%CI 0.72 to 0.99]; p=0.038) were the only predictors of a composite adverse blood product requirement/intervention outcome (Table 4). The odds of blood product requirement/intervention postnatally at 1st IUT (median HB at 1st IUT of 6.6g/dL) at 28 weeks were 43% and increased by a further 38% for delivery at 34 weeks. Conclusion: The earlier in gestation that IUTs are implemented, the higher the odds of a SB/NND; however the later the gestation at delivery, the greater the odds of the neonate requiring blood products post-partum. The greater the HB drop between the 1st and 2nd IUT, the shorter the 'time between the first IUT and delivery', which increases the odds of a SB/NND outcome. Disclosures Garbowski: Vifor Pharma: Consultancy, Membership on an entity's Board of Directors or advisory committees; Imara: Consultancy. Shehata:Ferring: Honoraria.

Hemolytic disease of the newborn caused by anti-U: A case report

Maternal red blood cell alloimmunization is an important cause of morbidity and mortality in the antepartum and neonatal periods. Typically, the serological diagnosis of Hemolytic Disease of the Fetus and Newborn (HDFN) includes a positive direct antiglobulin test on the infant’s red blood cells and the presence of an IgG red cell alloantibody in both maternal and cord sera.

Evaluation of a Decision Tree for Efficient Antenatal Red Blood Cell Antibody Screening.

Hemolytic disease of the fetus and newborn due to maternal red blood cell alloimmunization can have serious consequences. Because early detection enables careful monitoring of affected pregnancies, programs to routinely screen all pregnant women have been widely adopted. Due to the low prevalence of alloimmunization, these require large investments of resources to detect a small number of cases. We conducted a validation study of a decision tree developed in the Netherlands for determining whether to screen for alloimmunization. In a Swedish cohort, we compared the performance of that decision tree to two alternative models that used maternal characteristics, obstetric history, and transfusion history to identify high-risk women for screening or low-risk women who might be exempt from screening. The models were compared for predictive ability and potential reduction in the volume of screening. The decision tree applied to our study population identified 89% of alloimmunized women with a negative predictive value (NPV) of 99.7% by screening 62% of the population. To achieve the same NPV, our model exempting low-risk women captured 90% of alloimmunizations by screening 63% of the population. In contrast, the model identifying high-risk women for screening while maintaining a similar NPV captured 63% of alloimmunized women by screening 20% of the population. We validated that an existing decision tree for selecting women for maternal screening performed well in our population, identifying a large proportion of women who became alloimmunized, with a predictive performance almost identical to that of a more elaborate model.

Maternal red blood cell alloimmunization requiring intrauterine transfusion: a comparative study on management and outcome depending on the type of antibody.

The antibody primarily responsible for fetal anemia may influence treatment and prognosis. The primary objective was to compare ante- and postnatal management and the outcomes of maternal red blood cell (RBC) alloimmunizations according to the antibody involved. The secondary objective was to compare anti-D alloimmunizations according to associated number of antibodies. A single-center study from 1999 to 2015 including maternal RBC alloimmunizations requiring intrauterine transfusion (IUT) was conducted. Patients were classified according to the antibody involved: anti-D, other Rh (anti-c and anti-E), and anti-K1. Obstetric data, IUT characteristics, and neonatal outcome were compared. A specific study on the anti-D, when isolated or associated, was then conducted. There were 106 pregnancies included, with 77.4% having anti-D, 9.4% having another anti-Rh (Rh group), and 13.2% having anti-K1. No significant difference between the anti-D and Rh groups was found for management and prognosis. The hemoglobin level in the first IUT was higher in the anti-D group than in the Kell group (6.8 vs. 4.7 g/dL, p = 0.008). Newborns in the anti-D group had significantly higher bilirubin levels and phototherapy duration than those in the Kell group. The mean estimated daily decrease in hemoglobin and that between the first two IUTs were lower with an isolated anti-D, compared with anti-D associated with two antibodies (p = 0.04). Anti-K1 alloimmunizations seem to cause more severe fetal anemia than anti-D alloimmunizations. Moreover, a decrease in hemoglobin appears to be more rapid when anti-D is associated with other antibodies.

Acoustic radiation force impulse elastosonography of placenta in maternal red blood cell alloimmunization: a preliminary and descriptive study.

Maternal red blood cell alloimmunization is an important cause of fetal morbidity and mortality in the perinatalperiod, despite well-organized prophylaxis programs. The objective of the study was to evaluate placental elasticity by usingAcoustic Radiation Force Impulse (ARFI) in Rhesus (Rh) alloimmunized pregnant women with hydropic and nonhydropicfetuses and to compare those with healthy pregnant women. This case-control and descriptive studycomprised twenty-eight healthy pregnant women, 14 Rh alloimmunized pregnant women with nonhydropic fetuses, and 16 Rhalloimmunized pregnant women with hydropic fetuses in the third trimester of pregnancy. Placental elasticity measurementswere performed by ARFI elastosonography at the day of delivery. The maternal characteristics and neonatal outcomes of thepatients were also noted. The highest mean placental ARFI scores were observed in Rh alloimmunized pregnantwomen with hydropic fetuses (1.13 m/s) (p=0.001). Healthy controls and Rh alloimmunized pregnant women with nonhydropicfetuses had similar mean placenta ARFI scores (0.84 m/s, 0.88 m/s, respectively) (p<0.05). Based onthe present findings, the placenta becomes stiffer in Rh alloimmunized pregnancies complicated with hydrops fetalis. Theincreased placental ARFI scores may be a supplemental marker for adverse pregnancy outcomes, additional to Doppler evaluationof middle cerebral artery. This data should be confirmed with a large sample size and prospective studies by using serialmeasurements of ARFI elastosonography in maternal red blood cell alloimmunization.

Neonatal outcome after fetal anemia managed by intrauterine transfusion.

In-utero transfusion is now well under control and improves the survival of foetuses monitored for fetal anemia with a survival rate of more than 80%. The aim was to evaluate short-term neonatal outcome after fetal severe anemia managed by intrauterine transfusions. We did a retrospective study of all neonates born after management of severe fetal anemia (n = 93) between January 1999 and January 2013 in our regional center. The two main causes of anemia were maternal red blood cell alloimmunization (N = 81, 87%) and Parvovirus B19 infection (N = 10, 10.8%). In the alloimmunization group, phototherapy was implemented in 85.2% of cases with a maximum level of bilirubin of 114.4 ± 60.7 (mg/dl). Transfusion and exchange transfusion were, respectively, required in 51.9% and in 34.6% of cases. One neonate presented a convulsive episode, and we observed three neonatal deaths. In the parvovirus group, none of the child had anemia at birth and no management was necessary. Contemporary management of Rhesus disease is associated with encouraging neonatal outcomes. In case of Parvovirus infection, no specific management is necessary at. But, in all cases of fetal anemia, children should be followed up with particular attention to neurologic development. • In-utero transfusion is now well under control and improves the survival of fetuses monitored for fetal anemia. • Limited studies are available on the effect of IUT on postnatal outcome in infants with a history of fetal anemia. What is New: • Contemporary management of severe Rhesus disease is associated with encouraging neonatal outcomes. • The majority of infants can be managed with phototherapy and a limited number of top-up transfusions and exchange transfusions. In case of Parvovirus infection, the short-term neonatal outcome is excellent.

Associations of Rhesus and non-Rhesus maternal red blood cell alloimmunization with stillbirth and preterm birth.

Although the risks of adverse pregnancy outcomes associated with anti-D antibodies are well-recognized, much less is known concerning alloimmunization with other red blood cell antibodies detected during routine maternal screening. To date, most reports of adverse pregnancy outcomes associated with non-anti-D antibodies have been from small case studies. The aim of this study was to examine the associations of maternal alloimmunization with specific red blood cell antibodies and the risks of preterm birth and stillbirth in the Swedish population. All antibody screening, outcome and covariate data were obtained through linkages of Swedish national health and data registers. Follow-up in these population-based registers was available up to 31 December 2002. The final study sample consisted of 1,022,569 singleton births from 668,952 mothers during 1987-2002. In total, 1.3% of the 1,022,569 study pregnancies were alloimmunized. In adjusted logistic regression models, compared with having no antibodies, alloimmunization with anti-D, anti-E, anti-C and anti-c was associated with increased risk of both stillbirth and preterm birth. In addition, anti-Kell was associated with increased risk of preterm birth and anti-Lea with increased risk of stillbirth. Compared with firstborn children, risk of preterm birth associated with alloimmunization was greater in subsequent births In the largest study to date, alloimmunization with Rhesus, K- and -Lea red blood cell antibodies increased the risk of preterm birth and/or stillbirth. The association of anti-Lea with stillbirth was an unexpected finding. Further study of the consequences of non-anti-D alloimmunization is warranted.

Prenatal Diagnosis of Anoxic Cerebral Lesions Caused by Profound Fetal Anemia Secondary to Maternal Red Blood Cell Alloimmunization

The long-term neurological prognosis of severe fetal anemia is usually considered favorable, especially when fetal hydrops regresses after successful in utero transfusion. We report two cases of prenatally diagnosed fetal cerebral anoxic lesions associated with severe fetal anemia despite appropriate and successful treatment by in utero transfusion. The two pregnancies were terminated. Profound fetal anemia may cause anoxic lesions of the fetal brain that may be diagnosed prenatally. If new onset ventriculomegaly is observed on ultrasonography after in utero transfusion for severe fetal anemia, anoxic lesions could be suspected.

Doppler ultrasonographic evaluation of twins discordant for Rh alloimmunization

Minimally invasive methods have been used successfully to diagnose and treat maternal red blood cell alloimmunization. We present a case of a dizygotic twin pregnancy discordant for the Rh allele. Serial Doppler ultrasonography helped to diagnose the fetus that was at-risk for anemia caused by Rh alloimmunization and to direct treatment. (Am J Obstet Gynecol 2002;187:250-1.)

Human fetal bilirubin levels and fetal hemolytic disease

The development of secondary fetal anemia in association with maternal red blood cell alloimmunization requires hemolysis. In specimens obtained at the time of a clinically indicated cordocentesis, total and direct umbilical venous bilirubin was measured and the indirect umbilical venous bilirubin calculated in 43 antigen-positive and 30 control fetuses. Twenty-two (51%) of the antigen-positive fetuses had or subsequently developed severe anemia (hematocrit less than 30%). Umbilical venous total bilirubin (r = 0.47, p = 0.0008) and direct bilirubin (r = 0.520, p = 0.04) levels each rose with gestation. Indirect bilirubin did not vary significantly with gestation. Bilirubin was unrelated to hemoglobin. In contrast to the control fetuses, umbilical venous total bilirubin for antigen-positive fetuses was inversely related to hemoglobin (r = -0.57, p less than 0.0001) independent of gestational age (r = 0.53, p less than 0.0001) (multiple R of hemoglobin and gestational age for umbilical venous total bilirubin = 0.76, p less than 0.0001). Eighteen of 22 (82%) fetuses in whom anemia developed had an umbilical venous total bilirubin greater than or equal to 97.5 percentile compared with only eight of 21 (38%) fetuses in whom anemia did not develop (p = 0.009). In longitudinal study the umbilical venous total bilirubin frequently rose above normal weeks before the development of anemia. An umbilical venous total bilirubin greater than 3 mg/dl represented the warning line. Fifteen of 16 (94%) fetuses in whom either severe antenatal anemia or significant postnatal hyperbilirubinemia developed had an umbilical venous total bilirubin greater than 3 mg/dl. We conclude that the normal placental capacity for the transport of fetal bilirubin is exceeded in the face of enhanced fetal hemolysis. An elevated fetal bilirubin often precedes the development of antenatal anemia. The antigen-positive fetus with an elevated bilirubin is at high risk to develop anemia antenatally.

Human fetal bilirubin levels and fetal hemolytic disease

The development of secondary fetal anemia in association with maternal red blood cell alloimmunization requires hemolysis. In specimens obtained at the time of a clinically indicated cordocentesis, total and direct umbilical venous bilirubin was measured and the indirect umbilical venous bilirubin calculated in 43 antigen-positive and 30 control fetuses. Twenty-two (51%) of the antigen-positive fetuses had or subsequently developed severe anemia (hematocrit less than 30%). Umbilical venous total bilirubin (r = 0.47, p = 0.0008) and direct bilirubin (r = 0.520, p = 0.04) levels each rose with gestation. Indirect bilirubin did not vary significantly with gestation. Bilirubin was unrelated to hemoglobin. In contrast to the control fetuses, umbilical venous total bilirubin for antigen-positive fetuses was inversely related to hemoglobin (r = -0.57, p less than 0.0001) independent of gestational age (r = 0.53, p less than 0.0001) (multiple R of hemoglobin and gestational age for umbilical venous total bilirubin = 0.76, p less than 0.0001). Eighteen of 22 (82%) fetuses in whom anemia developed had an umbilical venous total bilirubin greater than or equal to 97.5 percentile compared with only eight of 21 (38%) fetuses in whom anemia did not develop (p = 0.009). In longitudinal study the umbilical venous total bilirubin frequently rose above normal weeks before the development of anemia. An umbilical venous total bilirubin greater than 3 mg/dl represented the warning line. Fifteen of 16 (94%) fetuses in whom either severe antenatal anemia or significant postnatal hyperbilirubinemia developed had an umbilical venous total bilirubin greater than 3 mg/dl. We conclude that the normal placental capacity for the transport of fetal bilirubin is exceeded in the face of enhanced fetal hemolysis. An elevated fetal bilirubin often precedes the development of antenatal anemia. The antigen-positive fetus with an elevated bilirubin is at high risk to develop anemia antenatally.

Management of fetal hemolytic disease by cordocentesis: II. Outcome of treatment

Forty-eight of 128 pregnancies complicated by maternal red blood cell alloimmunization (49%) received a total of 142 intravascular transfusions (range, 1 to 7) for treatment of severe anemia (hematocrit, ⪯30%). Thirteen fetuses (27%) had hydrops when therapy was initiated. The overall survival rate was 96%. Eighty-five percent of survivors received two or more transfusions before delivery. The mean gestational age at initiation of therapy was 28 weeks (range, 18 to 36 weeks). Bleeding from uterine and umbilical cord puncture sites was not of clinical significance. The most common complication was fetal bradycardia (8%). Simple intravascular transfuson resulted in the replacement of fetal red blood cells with adult red blood cells and suppression of fetal erythropoiesis. By the completion of the second transfusion, on average, <1% of circulating red blood cells were fetal. Within 3 weeks of the second transfusion, the mean reticulocyte count was <1%. The rate at which the fetal hematocrit declined after a transfusion (exclusive of the first) was inversely related to gestational age (r = −0.84, p < 0.0001), permitting a 4- to 5-week interval between transfusions after 32 weeks' gestation. A total of 78% of surviving neonates were delivered at term. Neonates transfused more than once antenatally required less phototherapy (75.8 ± 54 vs 165 ± 101 hours, p < 0.003) and, when delivered at term, fewer hospital days (4.8 ± 2 vs 8.6 ± 6 days, p = 0.01 ) compared with those transfused once. We conclude that the treatment of fetal anemia by intrauterine simple intravascular transfusion permits a term delivery in the majority of cases and is associated with high perinatal survival and low perinatal morbidity. Forty-eight of 128 pregnancies complicated by maternal red blood cell alloimmunization (49%) received a total of 142 intravascular transfusions (range, 1 to 7) for treatment of severe anemia (hematocrit, ⪯30%). Thirteen fetuses (27%) had hydrops when therapy was initiated. The overall survival rate was 96%. Eighty-five percent of survivors received two or more transfusions before delivery. The mean gestational age at initiation of therapy was 28 weeks (range, 18 to 36 weeks). Bleeding from uterine and umbilical cord puncture sites was not of clinical significance. The most common complication was fetal bradycardia (8%). Simple intravascular transfuson resulted in the replacement of fetal red blood cells with adult red blood cells and suppression of fetal erythropoiesis. By the completion of the second transfusion, on average, <1% of circulating red blood cells were fetal. Within 3 weeks of the second transfusion, the mean reticulocyte count was <1%. The rate at which the fetal hematocrit declined after a transfusion (exclusive of the first) was inversely related to gestational age (r = −0.84, p < 0.0001), permitting a 4- to 5-week interval between transfusions after 32 weeks' gestation. A total of 78% of surviving neonates were delivered at term. Neonates transfused more than once antenatally required less phototherapy (75.8 ± 54 vs 165 ± 101 hours, p < 0.003) and, when delivered at term, fewer hospital days (4.8 ± 2 vs 8.6 ± 6 days, p = 0.01 ) compared with those transfused once. We conclude that the treatment of fetal anemia by intrauterine simple intravascular transfusion permits a term delivery in the majority of cases and is associated with high perinatal survival and low perinatal morbidity.

Management of fetal hemolytic disease by cordocentesis: II. Outcome of treatment

Forty-eight of 128 pregnancies complicated by maternal red blood cell alloimmunization (49%) received a total of 142 intravascular transfusions (range, 1 to 7) for treatment of severe anemia (hematocrit, less than or equal to 30%). Thirteen fetuses (27%) had hydrops when therapy was initiated. The overall survival rate was 96%. Eighty-five percent of survivors received two or more transfusions before delivery. The mean gestational age at initiation of therapy was 28 weeks (range, 18 to 36 weeks). Bleeding from uterine and umbilical cord puncture sites was not of clinical significance. The most common complication was fetal bradycardia (8%). Simple intravascular transfusion resulted in the replacement of fetal red blood cells with adult red blood cells and suppression of fetal erythropoiesis. By the completion of the second transfusion, on average, less than 1% of circulating red blood cells were fetal. Within 3 weeks of the second transfusion, the mean reticulocyte count was less than 1%. The rate at which the fetal hematocrit declined after a transfusion (exclusive of the first) was inversely related to gestational age (r = -0.84, p less than 0.0001), permitting a 4- to 5-week interval between transfusions after 32 weeks' gestation. A total of 78% of surviving neonates were delivered at term. Neonates transfused more than once antenatally required less phototherapy (75.8 +/- 54 vs 165 +/- 101 hours, p less than 0.003) and, when delivered at term, fewer hospital days (4.8 +/- 2 vs 8.6 +/- 6 days, p = 0.01) compared with those transfused once. We conclude that the treatment of fetal anemia by intrauterine simple intravascular transfusion permits a term delivery in the majority of cases and is associated with high perinatal survival and low perinatal morbidity.

Management of fetal hemolytic disease by cordocentesis: I. Prediction of fetal anemia

Between January 1985 and November 1990, 128 pregnancies complicated by maternal red blood cell alloimmunization were referred to our Fetal Diagnosis and Treatment Unit. We examined the premise that an evaluation of fetal blood would accurately identify fetuses at risk of requiring antenatal transfusion therapy. Two hundred seventy-two diagnostic cordocenteses were performed. Criteria for the timing of repeat cordocenteses were developed retrospectively on the basis of the fetal hematocrit values, reticulocyte counts, and direct Coombs' test results of the first 84 pregnancies. These criteria were tested and confirmed prospectively on the next 44 pregnancies. On the basis of the first blood sample, four hematologic patterns (and their distributions) were identified in the 98 antigen-positive fetuses. Pattern 1: fetuses at low risk of having significant antenatal anemia (hematocrit &#x0026;#x0026;#x003C;30%) (n = 11,11 %). These fetuses had normal hematocrit values and reticulocyte counts coupled with negative or trace-positive direct Coombs' test. No fetus in this group had significant antenatal' anemia. Pattern 2: fetuses at intermediate risk of having anemia (n = 29, 31 %). Pattern 2 fetuses had normal hematocrit values and either direct Coombs' titers of more than trace ≤2+ and normal reticulocyte counts or low reticulocyte counts (<2.5th percentile for gestation). Twenty-one percent (n = '6) of fetuses in pattern 2 had significant antenatal anemia. Patterns 3 and 4: fetuses at greatest risk of having severe anemia. These fetuses had normal hematocrit values associated with either reticulocyte counts >97.5th percentile for gestation or a direct Coombs' test ≥3+ (pattern 3, n = 49, 50%) or both, or a mild anemia (>30% but <2.5th percentile for gestation) (pattern 4, n = 9, 10%). Eighty percent (n = 39) of fetuses with pattern 3 and 90% (n = 8) with pattern 4 developed a hematocrit value <30%. We conclude that evaluation of fetal hemolytic disease with a fetal blood specimen permits the identification,of fetuses at high risk of having antenatal anemia.