Pocket ObGyn – Hematologic Changes of Pregnancy / Alloimmunization
See Abbreviations
Plasma Volume
- by 40–50% of baseline plasma vol
- Plasma vol begins at ~6 w gest & continues until 30–34 w
RBC Mass
- 20–30% in RBC mass during Preg beginning at ~10 w gest
- 1000 mg iron req for Preg (RBCs – 500 mg, fetus – 300 mg, bleeding – 200 mg)
- Most common cause of anemia in Preg is iron deficiency
Leukocytes
- Plasma levels variable throughout Preg, WBC = 5000–12000/mL
- Physiologic leukocytosis in labor & puerperium, WBC = 14000–16000/mL
Coagulation System
- 5-fold increased risk of thromboembolic dz; absolute risk 1/1500 pregnancies
- risk from venous stasis (uterine mass effect), vessel wall injury, hypercoagulable state ( procoagulants; ¯ prot S; decreased fibrinolysis due to ¯ tPa)
- Coagulation factors normalize 2 w postpartum
Blood Loss with Delivery
- Avg EBL:Vaginal deliv = 500 mL; cesarean deliv = 1000 mL
- Cesarean hysterectomy = 1500 mL (nonurgent) & 2500 mL (emergent)
- Majority of bld loss w/i 1st hour after deliv ® ~80 mL lochia over next 72 h
Alloimmunization
Definition, Etiology, Epidemiology
- Mat antibodies to any fetal bld group factor inherited from father
- RhD Ag most commonly implicated; minor antigens include C, c, E, e, Kell
- Mat exposure to paternal Ag on fetal RBC ® Ab formation ® IgG crosses placenta & directs immune-mediated destruction of fetal RBCs
- 1 mL fetal bld may result in mat Ab formation; 2nd exposure ® anamnestic immune resp
- 8/1000 live births affected by alloimmunization; 10% prior to routine testing/ prevention
- Minor antigens present in ~2% of pregnancies
Clinical Manifestations
- Positive Ab screen on bld typing
- Fetal anemia ® hydrops fetalis (³2 of the following: Ascites, pleural effusion, pericardial effusion, skin edema, polyhydramnios)
- Fetal complications (death, hemolytic dz of newborn)
Screening/Diagnosis (see also Chap. 11)
- 1st OB visit: Mat bld type & Ab screen; consider rpt @ 28 w if RhD neg
- If mat RhD neg & paternity known ® obtain paternal bld type
- Anti-RhD Ab (+) ® indirect Coombs; Critical titer typically 1:8–1:32 (lab dependent)
- Prior affected Preg: risk fetal anemia; Ab titers do not correlate w/ severity
- FMH testing: Rosette – qualitative, K-B – If rosette + ® K-B used to determine dose of anti-RhD Ig.
- cfDNA: Fetal DNA in mat bld used to determine fetal RhD status
- RhD genotyping: Determines if RhD Ag gene present on 1 (heterozygote) or both (homozygote) chromosomes
- MCA Dopplers: Records PSV of MCA; PSV >5 MoMs for GA predictive of mod– sev fetal anemia
Figure 16.8 Management of alloimmunization in pregnancy
(From Moise KJ Jr. Management of rhesus alloimmunization in pregnancy. Obstet Gynecol. 2008;112(1):164–176. doi:10.1097/AOG.0b013e31817d453c)
Prophylaxis/Treatment
- Anti-RhD Ig ® 300 mcg neutralizes 30 mL whole bld (15 mL fetal RBCs) after FMH
- 50 mcg if <12 w gest; human serum-derived product; effect up to 12 w; max dose 1500 mcg/24 h; give w/i 72 h of indication for prevention of alloimmunization
- Weak RhD pos (Du) ® treat as RhD pos, ppx not indicated
- Rx ® deliv; intrauterine bld xfusion if remote from term
| Indications for anti-RhD Ig in RhD (–)/antibody (–) patient | ||
| Postdelivery (baby RhD +) | 24–28 w gest | 2nd/3rd trimester bleeding |
| Ectopic Preg | Amniocentesis | Chorionic villus sampling |
| Trauma | Threatened abortion | Cordocentesis |
| External cephalic version | IUFD | Molar gest |
Minor Antigens (Ag) (Obstet Gynecol 2006;108(2):457)
- Minor antigens present in ~2% of pregnancies
- Many may case RBC destruction; no prophylactic rx available; mgmt of sensitization is Ab dependent, but typically mirrors RhD
- Lewis & I most common ® do not cause erythroblastosis fetalis
- Anti-Kell Ab ® may cause sev anemia, follow w/ MCA Dopplers, titers unreliable
- Anti-RhD Ig indicated in RhD neg pts w/ minor antigens but no RhD antibodies