Induced Abortion and the Risk of Rh Sensitization

Abstract

Importance: While population-level data suggest Rh immunoglobulin is unnecessary before 12 weeks' gestation, clinical evidence is limited. Thus, guidelines vary, creating confusion surrounding risks and benefits of Rh testing and treatment. As abortion care in traditional clinical settings becomes harder to access, many people are choosing to self-manage and need to know if ancillary blood type testing is necessary.

Objective: To determine how frequently maternal exposure to fetal red blood cells (fRBCs) exceeds the most conservative published threshold for Rh sensitization in induced first-trimester abortion.

Design, setting, and participants: Multicenter, observational, prospective cohort study using high-throughput flow cytometry to detect circulating fRBCs in paired maternal blood samples before and after induced first-trimester abortion (medication or procedural). Individuals undergoing induced first-trimester abortion before 12 weeks 0 days' gestation were included. Paired blood samples were available from 506 participants who underwent either medical (n = 319 [63.0%]) or procedural (n = 187 [37.0%]) abortion.

Exposure: Induced first-trimester abortion.

Main outcomes and measures: The primary outcome was the proportion of participants with fRBC counts above the sensitization threshold (125 fRBCs/5 million total RBCs) after induced first-trimester abortion.

Results: Among the 506 participants, the mean (SD) age was 27.4 (5.5) years, 313 (61.9%) were Black, and 123 (24.3%) were White. Three of the 506 participants had elevated fRBC counts at baseline; 1 of these patients had an elevated fRBC count following the abortion (0.2% [95% CI, 0%-0.93%]). No other participants had elevated fRBC counts above the sensitization threshold after induced first-trimester abortion. The median change from baseline was 0 fRBCs, with upper 95th and 99th percentiles of 24 and 35.6 fRBCs, respectively. Although there was a strong association between the preabortion and postabortion fRBC counts, no other baseline characteristic was significantly associated with postabortion fRBC count.

Conclusions and relevance: Induced first-trimester abortion is not a risk factor for Rh sensitization, indicating that Rh testing and treatment are unnecessary before 12 weeks' gestation. This evidence may be used to inform international guidelines for Rh immunoglobulin administration following first-trimester induced abortion.

Figure 1.. Patient Recruitment in Study of First-Trimester Abortion

Participants were approached for inclusion if they were obtaining an abortion by medication or procedure at less than 12 weeks’ gestation. The most common reason for declining was not wanting to return for the postabortion blood draw. Thirty patients with a postabortion sample after 72 hours were included.

Figure 2.. Postabortion Levels of Fetal Red Blood Cell (fRBC) and Change From Preabortion Levels Stratified by Assay and Abortion Type

The bars in the histogram show the percentage for each optimization and abortion method group. The vertical blue line shows the sensitization threshold of 125 fRBCs/5 million cells. Only 1 participant had a postabortion level that exceeded 125 fRBCs/5 million cells.

Figure 3.. Fetal Red Blood Cell (fRBC) Count After Abortion by Gestational Age

Preoptimization of flow cytometry assay is associated with higher detected levels of postabortion fRBCs (P < .001), but gestational age is not (P = .14). See Table 2 for details. To display on a log scale (y-axis), a value of 1 was added to any participant with an fRBC level of 0/5 million cells. The solid black line at 125 fRBCs/5 million cells shows the threshold for sensitization. The orange, gray, and blue lines show median values for each optimization and abortion method group. The locally estimated scatterplot smoothing regression curve for the pooled data is show in black with gray shading representing the 95% CI.