When a Mother's Antibodies Attack Her Baby's Blood: What Today's Best Care Achieves and How a New Medicine's Dose Was Chosen

Two 2026 studies explained for families: the CLARITY study of early, severe hemolytic disease (Oepkes and colleagues) and the dose-selection work for the experimental medicine nipocalimab (Zhou and colleagues)

In a rare pregnancy condition, a mother's antibodies destroy her unborn baby's red blood cells, sometimes before 24 weeks. A 2026 study of 15 such pregnancies found that, even with expert care, 13 babies needed blood transfusions in the womb and all 14 newborns needed light treatment for jaundice. A second study explains how the dose of nipocalimab, an experimental antibody-blocking medicine, was chosen for a larger trial that is still under way.

Why this condition matters, and how families and doctors have faced it

Our immune system makes antibodies, small proteins that recognize and attack things that look foreign. Sometimes a pregnant woman's immune system makes antibodies against her baby's red blood cells, because the baby's red cells carry a blood-group marker that the mother's own cells lack. These antibodies cross the placenta and destroy the baby's red cells. Doctors call this hemolytic disease of the fetus and newborn, or HDFN ("hemolytic" means red cells breaking down). In severe cases the baby becomes very anemic (short of red blood cells) before birth, which can be life-threatening, and after birth the baby can develop very high levels of bilirubin, the yellow substance that causes jaundice and can harm the brain if it rises too far [1]. Most severe cases involve an antibody called anti-D, even though preventive injections given to mothers have made this form of the disease much less common [1][2].

For decades, the main treatment has been to give the baby a blood transfusion while still in the womb, called an intrauterine transfusion (IUT). An important advance was learning that ultrasound measurement of blood flow in the baby's brain can show when a baby is becoming anemic, without needing a needle [3]. In experienced centers, more than 95 out of 100 babies treated with transfusions in the womb now survive [4]. But when the disease starts very early, the risks rise: in one large Dutch center, 24 out of 100 babies were lost when the first transfusion was needed before 20 weeks, compared with 8 out of 100 when it came later [5]. The disease also tends to return in the next pregnancy, often earlier: in one national study, 86 out of 100 women who had needed transfusions before and became pregnant again with an affected baby needed transfusions again [6].

Some hospitals also give the mother infusions of intravenous immunoglobulin (IVIG), a product made from donated blood that may dampen the attack. It is not formally approved for this purpose, and the evidence comes from studies that looked back at past cases rather than from randomized trials, in which patients are assigned to treatments by chance [7][8]. Even so, 70 out of 100 experts on an international panel agreed that it should be used in the highest-risk pregnancies [9]. After birth, babies often need phototherapy (special blue light that breaks down bilirubin), sometimes an exchange transfusion (gradually replacing the baby's blood), and top-up transfusions for anemia in the weeks that follow [10]. Until recently, no study had carefully followed these pregnancies from the start to describe what modern care really achieves [11].

Two connected studies, one goal

The first study, by Oepkes and colleagues, is called CLARITY [11]. It was an observational study, meaning doctors followed pregnancies and recorded what happened without testing a new treatment. It was designed to match an earlier trial of the new medicine, called UNITY, so that the two could be looked at side by side. The second study, by Zhou and colleagues, used computer models to choose the dose of that medicine, nipocalimab, for a larger trial called AZALEA [12]. Both studies were paid for by Johnson & Johnson, the company developing nipocalimab, and company scientists helped write them [11][12]. An earlier In[Neo]Sight article looked at a different question, how blood tests in pregnancy can help predict which newborns need an exchange transfusion; you can read it here: HDFN burden and risk estimation.

What happened to the 15 pregnancies in CLARITY

Between August 2019 and September 2023, 15 pregnant women joined CLARITY at nine specialist HDFN hospitals in seven countries: Australia, Canada, the Netherlands, Spain, Sweden, the United Kingdom and the United States [11]. Every woman had already lost a baby or had a severely affected baby in an earlier pregnancy, and all received the standard care their hospital provided.

The researchers' main question was how many women would have a live baby at 32 weeks of pregnancy or later without ever needing a transfusion in the womb. The answer was 2 out of 15. Both of those women had received weekly IVIG at the same hospital, but because the study was small and not designed to test IVIG, it cannot tell us whether IVIG made the difference. Thirteen of the 15 babies needed transfusions in the womb, typically 4 each and as many as 11, sometimes starting as early as 13 weeks. One baby died before birth, at 16 weeks. Fourteen babies were born alive, half of them at full term, and most of the others between 32 and 36 weeks. In 4 of the 13 women who had transfusions, there was a complication such as a slowing of the baby's heart rate. Some women had side effects from IVIG, such as headache, and one developed meningitis (inflammation of the lining of the brain, not caused by infection) after her first infusion [11].

What happened to the babies after birth

All 14 newborns needed phototherapy for jaundice, most of them the more intensive kind. Three needed an exchange transfusion. Nine needed at least one ordinary blood transfusion in their first 12 weeks, and for six of them that first transfusion came two weeks or more after birth. Only three babies needed no transfusion at all. Thirteen babies were cared for in a neonatal intensive care unit (NICU), usually for about five days, and one baby with a liver problem stayed in hospital for 65 days [11]. This matches an earlier Dutch study in which 88 out of 100 babies who had transfusions in the womb needed transfusions in their first 3 months [13]. In short, today's care keeps most babies alive, but nearly all of them still need invasive treatment before or after birth.

A new approach: blocking the antibodies' ferry across the placenta

Antibodies reach the baby by riding on a carrier called FcRn, a kind of ferry in the placenta, and the same receptor keeps antibodies circulating longer in the mother's blood [14][12]. Nipocalimab is a laboratory-made antibody that blocks FcRn. In the UNITY trial, an early-stage (phase 2) study carried out at several hospitals in different countries, 7 out of 13 pregnancies treated with weekly nipocalimab reached 32 weeks or later without a transfusion in the womb [15][16]. It is tempting to compare 7 out of 13 with CLARITY's 2 out of 15, but these were separate small studies, not a fair head-to-head test, so the difference cannot be taken as proof that the medicine works.

How the dose for the larger trial was chosen

UNITY tried two weekly doses, 30 and 45 milligrams for every kilogram of the mother's body weight. Zhou and colleagues built computer models from the UNITY blood tests and earlier studies in healthy volunteers [12]. A key problem is that women gain weight during pregnancy, so a dose calculated at the start can become too small. The models predicted that the higher dose, recalculated as the mother's weight changes, would keep the receptor fully blocked in more than 95 out of 100 women even if a weekly dose was given 3 days late. With the lower dose, this fell to about 71 out of 100. Both doses lowered the mother's antibody levels by about 80% to 85%, and the medicine cleared from the body within one to two weeks after the last dose. The researchers therefore recommended the higher dose for AZALEA, while stressing that a computer model cannot prove that a medicine works or is safe [12].

What about the baby's own protection against infection?

Before birth, babies normally receive protective antibodies from their mothers, and nipocalimab lowers these too. In follow-up of the UNITY babies, antibody levels were low at birth but rose into the normal range over time in all but one child, 6 of the 7 children tested had protective levels against diphtheria and tetanus, and no unusual or unexpected illnesses were seen [17]. In UNITY, babies with very low antibody levels at birth were given an IVIG infusion as a precaution [12].

What this means for families today

Nipocalimab is not approved for this condition; it is still being tested [12]. The AZALEA trial plans to include about 120 women at around 50 specialist centers worldwide, with two out of every three receiving nipocalimab and one receiving a placebo (a dummy treatment) [16]. If you are affected, your specialist team can tell you whether a trial is open near you. Whatever the treatment before birth, it helps to know that phototherapy and transfusions are likely after birth, and that anemia can appear weeks later, so follow-up visits matter. Experts suggest checking babies until 2 to 3 months of age [9].

What researchers are working on next

The AZALEA trial is still going on, and its results are not yet available [12]. That trial, in which neither families nor doctors know who receives the medicine, should show whether nipocalimab truly prevents severe anemia in the womb, and how safe it is for mothers and babies. Researchers will also keep studying how babies' immune systems develop after treatment [17].

The bottom line

CLARITY shows that expert care saves most babies with this severe, early disease, but almost all still need transfusions in the womb and treatment after birth [11]. The companion study explains why the higher weekly dose was chosen for the AZALEA trial [12]. Whether that medicine changes the story for families is the question AZALEA is now answering.

References

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