Updates in the Prenatal Management of Prevention of Alloimmunization

In the November 2024 issue of NeoReviews, we present a case involving RhD alloimmunization of a pregnant woman and the implications for her neonate, as part of our monthly Maternal-Fetal Case Series. Historically, hemolytic disease of the newborn was a leading cause of fetal and neonatal mortality before the development of Rh immune globulin in the 1970s.¹  Rh immune globulin (RhIG), with brand names including Rhogam, WinRho, and Rhophylac, have been highly impactful in reducing the likelihood of alloimmunization from RhD and the sequelae of hemolytic disease of the newborn. A dose of Rh immune globulin given to all Rh-negative pregnant women universally in the third trimester and postpartum for those with confirmed Rh-positive neonates reduced the risk of RhD alloimmunization to 0.4-0.2%.¹ Currently, the standard of care by the American College of Obstetricians-Gynecologists is to provide Rh immune globulin to all Rh-negative pregnant women at 28 weeks’ gestation and again postpartum if the neonate is Rh positive.¹

The use of RhIG, while impactful at reducing the risk of isoimmunization against RhD, is limited, costly, and an irradiated blood product with a theoretical risk of transmitting blood-borne viruses.¹ The supply is therefore dependent on blood donors. Until recently, without invasive diagnostic testing such as amniocentesis or cordocentesis to assess fetal blood type, there was not a validated non-invasive means of assessing fetal Rh status. Therefore, with the inability to reliably detect Rh-negative fetuses, all pregnant Rh-negative people are recommended to receive Rh immune globulin during pregnancy to reduce the risk of alloimmunization without a means of assessing whether their fetus is Rh negative and therefore not require RhIG. This non-targeted approach of RhIG administration to all Rh-negative pregnant women leads to unnecessary prophylaxis of those women carrying fetuses who are also Rh negative. 

Non-invasive prenatal screening (NIPT) is cell-free DNA technology that uses multiplex polymerase chain reaction (PCR) next-generation sequencing based screening assay that utilizes primers to regions of the RhD gene and pseudogene. This technology has emerged as a reliable technology for screening Rh-negative pregnancies to evaluate whether the fetus is anticipated to be Rh positive or Rh negative. NIPT allows for detection of singleton fetuses with Rh-negative blood type who would therefore pose no risk of causing alloimmunization to a mother who similarly has Rh-negative blood. The use of NIPT technology is estimated to decrease the use of RhIG by 40% by identifying pregnancies with a Rh-negative fetus when RhIG is not necessary during pregnancy.^1,2 This technology is similar to NIPT for select fetal aneuploidies, microdeletions, microduplications, and other genetic point mutations.³ NIPT is not diagnostic without amniocentesis and is therefore considered a screening test, although data show that the sensitivity and specificity of the test demonstrating high reliability of the fetal Rh status.

In 2024, a national shortage of the RhIG supply prompted further consideration of non-invasive prenatal cell-free DNA (NIPT) technology in certain clinical situations to further delineate which pregnant individuals have a fetus with RhD negative blood type and therefore do not need RhIG administration in pregnancy. This recommendation by the American College of Obstetricians-Gynecologists was expected to help conserve the limited supply of RhIG.³

The validation of NIPT screening is leading to changing recommendations regarding the use of RhIG in the US. Recent research provided clinical validation in a racially diverse Rh-negative cohort that next generation sequencing NIPT correctly identified 356 of 356 Rh-positive fetuses, and 295 of 297 Rh-negative fetuses (2 false positive results). There were no false negative results and the cell-free DNA screening correctly identified 3 of 3 fetuses with the RhD pseudogene that is more prevalent in individuals with African descent.²

Fortunately, the recent study⁴ on NIPT did not demonstrate any false negative results indicating that there were no pregnant people who missed receiving a dose of RhIG during pregnancy when it was warranted for a Rh-positive fetus. While pregnant people should expect a test with the highest negative predictive value to avoid false negative results, a diagnostic test with neonatal cord blood typing is performed for all neonates born to Rh-negative individuals. Therefore, the unexpected false negative result on cell-free DNA screening will be determined by neonatal cord blood genotyping by a hospital’s blood bank. Fortunately, the majority of alloimmunization occurs not during pregnancy but at the time of birth.¹ If there was an Rh-positive fetus who was not identified prenatally by NIPT screening, RhIG could be provided after birth to the Rh-negative birthing person to minimize the chance of alloimmunization if the neonate was confirmed Rh positive from the neonatal cord blood.   

Use of universal NIPT screening for RhD has not yet been endorsed by the American College of Obstetricians-Gynecologists as of this writing; prior endorsement of use of this technology was tempered by cost analysis and concerns over inconclusive results with the Rh pseudogene that is more commonly seen in individuals with African background. Given the latest publication with successful identification of the Rh pseudogene combined with endorsement of use of NIPT technology for those pregnant people who already are alloimmunized and for those who decline amniocentesis,⁵ further analysis may lead to more universal use of NIPT technology for RhD screening in pregnancy.   

In the case presented in the November 2024 edition of NeoReviews, the Rh-negative pregnant patient with a high antibody titer underwent amniocentesis to determine the fetal blood type, which was Rh-positive. A potential option for this patient could have also been to utilize NIPT technology, which reliably would have determined the same information in a non-invasive fashion.

References

1. Prevention of Rh D alloimmunization. Practice Bulletin No 181. American College of Obstetricians and Gynecologists. Obstet Gynecol 2017; 130:e57.70.
2. Gilstrop Thompson M, Xu W, Moore B, Wang T, Sun N, Pewar H, Avent ND, Vernaza A, Acosta F, Saben JL, Souter V, Parmar S, Sengupta U, Altug Y, EmBree J, Cantos C, Kotwaliwale C, Babiarz J, Zimmermann B, Swenerton R, Meltzer JT. Clinical Validation of a Prenatal Cell-Free DNA Screening Test for Fetal RHD in a Large U.S. Cohort. Obstet Gynecol. 2024 Nov 26. doi: 10.1097/AOG.0000000000005794. Epub ahead of print. PMID: 39591628.
3. Gandhi M, Shields A, American College of Obstetricians and Gynecologists. Rho(D) Immune globulin Shortages. July 9, 2024. https://www.acog.org/clinical/clinical-guidance/practice-advisory/articles/2024/03/rhod-immune-globulin-shortages
4. Moise KJ Jr. The use of free DNA for fetal RHD genotyping in the Rh negative pregnant patient-the time has come. Am J Obstet Gynecol. 2024 Aug 16:S0002-9378(24)00840-8. doi: 10.1016/j.ajog.2024.08.017. Epub ahead of print. PMID: 39153534.
5. American College of Obstetricians and Gynecologists. ACOG Clinical Practice Update: Paternal and Fetal Genotyping in the Management of Alloimmunization in Pregnancy. Obstet Gynecol. 2024; 144(2): e47-e49.
6. American College of Obstetricians and Gynecologists. ACOG Clinical Practice Update: Rh D Immune Globulin Administration After Abortion or Pregnancy Loss at Less than 12 weeks Gestation. Obstet Gynecol. 2024; 144(6): e140-e143.