Hemolytic disease of the fetus and newborn

17 min

Explain how maternal IgG and newborn bilirubin clearance produce HDFN findings
Recognize suppression of erythropoiesis in anti-K-associated fetal anemia
Call a maternal titer change only when the same method retests the earlier sample

Try first

A mother has anti-D, and her D-positive baby's cord bilirubin is 3.4 mg/dL. At 18 hours of age the bilirubin is 11.8 mg/dL and still rising. Why does bilirubin climb after delivery?

The anti-D destroys red cells faster once the baby is born

The next section explains it.

The placenta no longer passes bilirubin to the mother's liver, and the newborn liver conjugates it slowly

Right. The next section explains why.

The newborn starts making its own anti-D

The next section explains it.

I don't know yet

The next section explains it.

Get the idea

Maternal IgG reaches fetal red cells

Only IgG crosses the placenta. The neonatal Fc receptor carries it, and transfer rises through the second trimester to a peak near term.¹ IgG against an antigen the fetus inherited from the father coats fetal red cells. Macrophages in the fetal spleen clear the coated cells, and anemia follows.²

Before birth, unconjugated bilirubin from the hemolysis crosses the placenta, and the mother's liver conjugates it. After delivery that route ends. The newborn liver conjugates poorly, so bilirubin rises in the first hours and days.^2,3 Maternal IgG has a half-life of about 25 days, so hemolysis continues after birth.¹

Anti-K also stops production

Kell antigens are present on erythroid precursors in the marrow. Anti-K suppresses red-cell production as well as destroying circulating cells.⁴ Less hemolysis makes less bilirubin, and the anti-K titer relates less predictably to fetal anemia. Both can understate how anemic the fetus is. The laboratory reports anti-K to the obstetric service promptly.^5,6 A 2025 guideline recommends a critical titer of 4 or more for anti-K and 16 or more for most other antibodies. Each laboratory validates the critical titer for its own method.⁶

A titer change needs the same method

The titer is the reciprocal of the highest dilution that still reacts. It follows the amount of antibody able to cross the placenta. Titers from different methods or laboratories are not interchangeable.⁵ To call a change:

Titrate the new sample by the laboratory's validated method.
Titrate the stored earlier sample alongside it, in the same run.
Count the doubling dilutions between them. A fourfold change is two doubling dilutions, for example 16 to 64.
Judge the change against the laboratory's significant-change criterion.

Cold-reacting IgM antibodies such as anti-Le^a, anti-Le^b, anti-I and anti-P1 usually need no HDFN follow-up.²

References

Palmeira P, Quinello C, Silveira-Lessa AL, Zago CA, Carneiro-Sampaio M. IgG placental transfer in healthy and pathological pregnancies. Clin Dev Immunol. 2012;2012:985646. doi:10.1155/2012/985646
Bloch EM, Campbell-Lee S, McKenna DH Jr, Montemayor-Garcia C, Schwartz J, Shaz B, Storry J, eds. Technical Manual. 22nd ed. AABB; 2026.
Kemper AR, Newman TB, Slaughter JL, et al. Clinical practice guideline revision: management of hyperbilirubinemia in the newborn infant 35 or more weeks of gestation. Pediatrics. 2022;150(3):e2022058859. doi:10.1542/peds.2022-058859
Vaughan JI, Manning M, Warwick RM, Letsky EA, Murray NA, Roberts IAG. Inhibition of erythroid progenitor cells by anti-Kell antibodies in fetal alloimmune anemia. N Engl J Med. 1998;338(12):798-803. doi:10.1056/NEJM199803193381204
Regan F, Veale K, Robinson F, et al. Guideline for the investigation and management of red cell antibodies in pregnancy: a British Society for Haematology guideline. Transfus Med. 2025;35(1):3-23. doi:10.1111/tme.13098
Moise KJ Jr, Markham KB, Spinella PC, et al. A clinical practice guideline for the management of pregnancy alloimmunized to red blood cell antigens. JAMA Netw Open. 2025;8(11):e2544649. doi:10.1001/jamanetworkopen.2025.44649

Watch one

Priya Nair, 31, MRN 4418203, is referred at 24 weeks of pregnancy with anti-c. The referring hospital reported a titer of 16 at 18 weeks by its own tube method, and sent that sample with the referral. Your laboratory titrates today's sample and gets 64. Its validated critical titer is 16, and its procedure counts a fourfold change as significant.

Has the titer risen significantly?

Confirm the specificity. Anti-c reacts at the antiglobulin phase and is a recognized cause of HDFN, so it is titrated.
Titration and follow-up are for IgG antibodies that can cause HDFN.
Check where each titer came from. The 16 is another laboratory's result by another method, so it cannot be compared with today's 64.
Titers from different methods or laboratories are not interchangeable.
Thaw the stored 18-week sample and titrate it alongside today's. The 18-week sample reacts through 1:32, a titer of 32. Today's sample again gives 64.
A parallel test puts both samples through the same method in the same run.
Count the steps. From 32 to 64 is one doubling dilution, a twofold change.
Each doubling dilution is a twofold step.
Report today's titer of 64 with the method and the parallel result. Both titers are at or above the critical titer of 16.
The laboratory reports the titer and the method, and the obstetric service decides on fetal surveillance.

No significant rise. Tested side by side, the titer went from 32 to 64, a twofold change below the fourfold criterion. The apparent fourfold rise from 16 came from comparing two methods.

Your turn

Problem 1 of 3

A patient at 26 weeks of pregnancy has a reactive antibody screen and a nonreactive autocontrol. Cross out what the panel rules out, then name the antibody.

Antibody panel: + the cell carries the antigen, 0 it lacks it
Cell	D	C	E	c	e	K	k	Fy^a	Fy^b	Jk^a	Jk^b	M	N	S	s	P1	Le^a	Le^b	37	AHG
1	+	+	0	0	+	0	+	+	0	+	0	+	0	+	0	+	0	+	0	0
2	+	0	+	+	0	0	+	0	+	0	+	+	+	0	+	+	+	0	w+	3+
3	0	+	0	+	+	+	+	+	+	+	+	0	+	+	+	0	0	+	0	0
4	0	0	0	+	+	0	+	0	+	0	+	+	+	0	+	+	+	0	0	0
5	+	0	0	+	+	0	+	+	+	+	0	0	+	+	0	0	0	+	0	0
6	0	0	+	+	+	0	+	+	0	+	+	+	+	+	+	+	0	+	0	2+
7	+	0	+	+	0	+	+	+	+	0	+	+	0	+	+	+	0	0	w+	3+
8	+	+	0	+	+	0	+	0	+	+	+	+	+	0	+	+	0	+	0	0
9	+	+	+	+	+	0	+	+	0	+	+	+	0	+	+	0	0	+	0	2+
10	0	0	0	+	+	+	+	+	+	+	+	+	+	+	+	+	0	+	0	0
11	+	0	+	+	+	0	+	0	+	0	+	+	+	+	+	+	+	0	0	2+
Autocontrol																			0	0

Hint

Start with the nonreactive cells. For C, c, E, e, Kidd, Duffy and MNS, cross out only on a nonreactive cell that is homozygous for the antigen.
Compare the reactive cells' strengths. Look for an antigen whose double-dose cells react more strongly than its single-dose cells.

Show the answer and the reasoning

Cell 1 is nonreactive and rules out D, C, e, k, Fy^a, Jk^a, M, S, P1, Le^b.

Cell 3 is nonreactive and rules out e, K, k, N, Le^b.

Cell 4 is nonreactive and rules out c, e, k, Fy^b, Jk^b, s, P1, Le^a.

Cell 5 is nonreactive and rules out D, c, e, k, Jk^a, N, S, Le^b.

Cell 8 is nonreactive and rules out D, e, k, Fy^b, s, P1, Le^b.

Cell 10 is nonreactive and rules out c, e, K, k, P1, Le^b.

Anti-E

Every reactive cell is E-positive and every nonreactive cell is E-negative. The E+e− cells 2 and 7 react 3+, and the E+e+ cells react 2+, the dosage anti-E often shows. Anti-E is IgG and can cause HDFN, so it is titrated and followed. Type her red cells for E to confirm she is E-negative.

Review Antibody investigation

Problem 2 of 3

Why can anti-K produce severe fetal anemia with less bilirubin elevation than expected from hemolysis alone?

Anti-K increases fetal red-cell production

Incorrect. Anti-K can suppress erythroid precursors, reducing the marrow response.

Anti-K prevents IgG from crossing the placenta

Incorrect. The disease depends on maternal IgG reaching fetal circulation. Anti-K does not block that transfer.

Anti-K can suppress erythroid precursors as well as destroy circulating cells

Correct. Kell expression on erythroid precursors allows reduced production to contribute to anemia. Bilirubin trends can understate its severity.

Hint

Ask where Kell antigens are carried besides mature red cells.
Follow what fewer new red cells means for the amount of hemoglobin broken down into bilirubin.

Review Disease-associated antibody specificities

Problem 3 of 3

A patient with anti-E transfers her care at 30 weeks. The referring hospital reported a titer of 8 by gel at 22 weeks and kept no sample. Your laboratory's tube method gives a titer of 32 today. How does the laboratory report the titer?

As a fourfold rise from 8 to 32

The two titers come from different laboratories and methods. Their difference can come from the method alone.

Compared titers from different methods as a true change

Titers from different methods or laboratories are not interchangeable, so a rise from 16 in one laboratory to 64 in another may reflect the method alone. A fourfold change counts when the same method tests the stored earlier sample in parallel with the new one.

As unchanged, because gel titers read one dilution higher than tube titers

No fixed conversion links two methods. A change is judged only when one method tests both samples.

Retitrate today's sample by gel and compare it with the 8

A gel method in another laboratory is still a different method and run. Only a parallel test of both samples shows a true change.

As a titer of 32 by this method, with the change unassessable, and store today's sample

Without the earlier sample, no parallel test is possible. Today's stored sample becomes the one titrated alongside the next.

Review Antibody identification in pregnancy

Use it

Lena Kowalski, 29, MRN 3307416, is 20 weeks into her second pregnancy. She has never been transfused.
Her antibody screen reacts at the antiglobulin phase, and her red cells type K-negative.
The laboratory titrates her antibody by its validated tube method and freezes each sample.

Antibody panel: + the cell carries the antigen, 0 it lacks it
Cell	D	C	E	c	e	K	k	Fy^a	Fy^b	Jk^a	Jk^b	M	N	S	s	P1	Le^a	Le^b	37	AHG
1	+	+	0	0	+	0	+	+	0	+	0	+	0	+	0	+	0	+	0	0
2	+	0	+	+	0	+	+	0	+	0	+	0	+	0	+	+	+	0	w+	3+
3	0	+	0	+	+	0	+	0	+	0	+	0	+	0	+	0	+	0	0	0
4	0	0	0	+	+	0	+	+	+	+	+	+	+	+	+	+	0	+	0	0
5	+	0	+	+	0	0	+	+	+	+	+	+	+	+	+	+	0	+	0	0
6	0	0	+	+	+	+	+	+	0	+	+	+	+	0	+	0	0	+	0	2+
7	+	+	0	+	+	0	+	+	+	0	+	+	+	+	+	+	0	0	0	0
8	0	0	0	+	+	+	+	0	+	+	0	+	0	+	+	+	0	+	w+	3+
9	+	+	0	0	+	0	+	0	+	+	+	0	+	+	0	+	+	0	0	0
10	+	+	0	+	+	+	+	+	+	+	0	+	+	+	+	0	0	+	0	2+
11	+	0	0	+	+	0	+	+	0	0	+	+	+	0	+	+	0	+	0	0
Autocontrol																			0	0

Decision 1 of 3

Her panel is below, and her titer is 8. What does the laboratory report?

A Lewis antibody, which needs no follow-up in pregnancy

Cell 3 carries Lea and cell 1 carries Leb, and both are nonreactive. The reactive cells 2, 6, 8 and 10 share K, and the reactions are at the antiglobulin phase.

Anti-K with a titer of 8, reported promptly to the obstetric service

Only the K-positive cells react, and she types K-negative. Anti-K suppresses fetal red-cell production, so a low titer can understate fetal anemia, and a 2025 guideline sets its critical titer at 4.

Anti-K with a titer of 8, below the usual critical titer of 16, repeated in 4 weeks

The specificity is right. The threshold of 16 fits most antibodies, and anti-K is reported promptly because its titer relates less predictably to fetal anemia.

Judged anti-K severity by titer and bilirubin

Kell antigens sit on erythroid precursors, so anti-K suppresses red-cell production as well as destroying circulating cells. Less hemolysis means less bilirubin, and the titer relates less predictably to anemia, so both can understate the severity of fetal anemia.

Review Disease-associated antibody specificities

Decision 2 of 3

At 26 weeks her new sample titrates to 32. The frozen 20-week sample, tested in the same run, gives 8. How does the laboratory report the change?

A twofold rise

From 8 to 16 is one doubling dilution, and from 16 to 32 is a second. Two doubling dilutions make a fourfold change.

No reportable change, because both titers are above 4

The change itself is reported. A rise in a titer already past the critical titer still tells the obstetric service the antibody is increasing.

A fourfold rise, two doubling dilutions, reported promptly

Both samples went through the same method in one run, so the change is real. From 8 to 32 meets a fourfold significant-change criterion.

Review Antibody identification in pregnancy

Decision 3 of 3

Her baby is born at 37 weeks, K-positive with an IgG-positive DAT. The cord bilirubin is 2.9 mg/dL, and at 20 hours it is 10.6 mg/dL. Why was the cord value low?

Hemolysis started only after delivery

Maternal IgG coated fetal cells for weeks before birth. The hemolysis was already there, and the placenta carried its bilirubin away.

Assumed bilirubin handling is unchanged after delivery

Before birth, unconjugated bilirubin from fetal hemolysis crosses the placenta and the mother's liver clears it. After delivery that route ends and the newborn liver conjugates poorly, so bilirubin rises after birth as maternal IgG keeps destroying red cells.

Cord blood dilutes the baby's bilirubin

Cord blood is the baby's own blood. Its bilirubin reflects what the placenta left behind.

Before birth the placenta passed bilirubin to the mother's liver

The mother's liver conjugated the fetal bilirubin. After birth the newborn liver clears it alone, and bilirubin rises as long as maternal anti-K persists.

Review Maternal antibody transfer to the fetal circulation

The clue that settled this case is the anti-K specificity. It made a titer of 8 a prompt report to the obstetric service, and the frozen samples let the laboratory call the later rise with confidence.

Keep

Sources checked September 27, 2026

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Hemolytic disease of the fetus and newborn

Try first

Get the idea

Maternal IgG reaches fetal red cells

Anti-K also stops production

A titer change needs the same method

Watch one

Your turn

Use it

Results

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