When a Baby's Heartbeat Is Too Slow Before Birth

Four recent studies from Brazil, China, Tunisia and Türkiye on a rare condition in which a mother's own antibodies slow her baby's heart — and what they mean for families

In a small number of pregnancies, antibodies made by the mother's immune system cross the placenta and damage the part of the baby's heart that sets its rhythm, leaving the heartbeat permanently slow. Four studies published in 2025 and 2026 looked at who is most at risk, what these babies are like at birth, and how a pacemaker can be fitted safely even in very small newborns. Together they show that the condition is manageable, and that most of the difference is made by planning ahead.

A problem that starts with the mother's immune system

Some people's immune systems make proteins called antibodies that mistakenly target their own tissues. Two of these, known by the shorthand names anti-Ro and anti-La, are common in autoimmune conditions such as lupus and Sjögren's syndrome. Many women who carry them feel completely well and have no diagnosis at all — the antibodies are simply there.

During pregnancy, antibodies of this type are carried across the placenta into the baby's bloodstream on purpose: this is how a newborn borrows its mother's immunity for the first months of life. Anti-Ro and anti-La cross too, and in a small proportion of pregnancies they settle in a very specific place — the atrioventricular node, a cluster of cells that relays the electrical signal from the upper chambers of the baby's heart to the lower ones. Inflammation there is followed by scarring, and the relay stops working. Doctors call the result congenital heart block. "Congenital" means present at birth; "block" means the electrical message is interrupted.

The timing is narrow and consistent. Most cases develop between about 18 and 26 weeks of pregnancy. Once the scar has formed, it does not heal, which is why so much attention goes into predicting and detecting the problem rather than repairing it afterwards.

How families used to face this, and what changed

For decades this condition was recognised only after a baby was born with an unexplained slow pulse, or after a routine scan late in pregnancy picked up a heart rate far below the usual 120 to 160 beats per minute. A large registry set up in the United States gave families the first reliable numbers: roughly two in every hundred antibody-exposed pregnancies are affected, but once one baby has had heart block, the chance in a later pregnancy rises to about one in five [1]. Later work from the same registry, and a European study of 175 affected pregnancies, described what happened to those babies and which of them did worst [2][3]. Reviews written since then made clear that the heart muscle and valves can be affected as well as the electrical system [4][5][6].

Doctors then tried to catch the problem early enough to stop it. The idea was that a mild, partial block would appear first and could be reversed with a steroid medicine given to the mother. A careful study called the PR Interval and Dexamethasone Evaluation — PRIDE for short — followed 98 pregnancies with frequent heart scans and found that this hope was mostly misplaced: complete block appeared without any warning sign, sometimes within a week of a completely normal scan [7]. The response was to check far more often. Mothers were taught to listen to the baby's heart rhythm at home with a handheld device [8], and a large North American study known as STOP BLOQ combined that with blood tests measuring how much antibody the mother carried. Among 413 pregnancies, 63% had high antibody levels, and heart block occurred in 3.8% of those — and in none of the women with low levels [9]. A separate trial showed that hydroxychloroquine, a long-established and inexpensive tablet, roughly halved the chance of the problem recurring when started before ten weeks of pregnancy [10], though a 2026 review concluded there is still not enough evidence to know whether it prevents a first case [11].

What the four new studies found

The first study followed 182 pregnancies over twenty years at a university hospital in São Paulo, Brazil [12]. Thirteen babies developed complete heart block. The mothers of those babies had much higher antibody levels than the others — roughly five times as much anti-Ro and fifteen times as much anti-La — and were more likely to have been recently diagnosed, to have active disease, and to have had a previously affected baby. Mothers who had been taking hydroxychloroquine before becoming pregnant were less often affected. The authors are careful, and so should readers be: these are laboratory numbers from one hospital's test, so the pattern travels but the specific figures do not.

The second study, from four hospitals in eastern China, described 91 newborns with neonatal lupus — the umbrella term for what these antibodies do to a baby [13]. Fifteen had heart block, and most of those had a partial rather than a complete block, which means they would not have looked obviously unwell. Babies with heart block tended to have higher levels of an enzyme called creatine kinase, released when muscle including heart muscle is under strain, and were more likely to have low red blood cell counts. The most reassuring finding was about everything other than the heart: rashes, blood count changes and liver abnormalities all faded on their own by the age of two, as the mother's antibodies gradually disappeared from the child. The heart block did not fade, and four of the five children with a complete block received a pacemaker at around one year of age when they became too easily tired to keep up.

The third study followed eight babies over six years at a specialist centre in Tunis, Tunisia [14]. All eight were spotted before birth, between 20 and 25 weeks. Only one mother knew she had an autoimmune condition — for the others, the baby's slow heartbeat was the first clue. The account is unusually honest about practical obstacles: a medicine normally used to lift a newborn's heart rate had not been available in the country for years, and pacemakers had to be imported with official approval, so some children waited months. Three babies did not survive: one before birth, in a pregnancy complicated by severe fluid build-up, one at three days old, and one at two months.

The fourth study, from Türkiye, tackled the problem those delays create [15]. Fitting a pacemaker in a premature baby weighing under 2500 grams is technically difficult, and many centres wait. This team instead used a two-step approach: temporary pacing wires placed through a small incision below the breastbone to stabilise the baby, then a permanent pacemaker once the baby had grown, at a median of 18 days old. The smallest permanent device went into a baby of 2200 grams. All eight children were alive and well after roughly two years, with no infections and no broken wires. One baby whose heart pumped poorly at first improved substantially once the pacing wire was moved to a different part of the heart.

What this means for families

If you are pregnant and known to carry anti-Ro or anti-La antibodies, the most useful conversations happen before conception or in the first weeks — about your antibody levels, about whether hydroxychloroquine is right for you, and about how often your baby's heart will be checked. If a slow heartbeat is found, ask where delivery is planned and whether the hospital has both the medicines and the pacemaker equipment on hand. That single question is what separated the smoother courses from the delayed ones in these studies.

It is also worth knowing what the first days usually involve. A baby with a suspected heart block will have an electrocardiogram, a painless recording of the heart's electrical activity, and an ultrasound scan of the heart to check how well it is pumping and whether the valves are normal. Some babies need nothing more than observation; others need medicines to support the circulation, and a few need a pacemaker urgently. Which group a baby falls into depends less on the antibodies than on how fast the heart is beating and how well the baby's body is coping with that rate.

If your baby is born with heart block, two things are worth holding onto. The heart rhythm problem is permanent, but it is treatable, and modern pacing in newborns has good results even in babies born very early [15]. And the other features of neonatal lupus — the rash, the blood counts, the liver tests — are temporary, because they are caused by antibodies that will leave your baby's body over the first year [13].

What researchers are working on next

Three questions are being actively pursued. The first is whether very frequent monitoring, with treatment given within hours, can genuinely stop a partial block from becoming complete; a 2026 report described exactly that happening in one pregnancy [16], and another team has begun giving antibody-blocking treatment directly to the fetus [17]. The second is whether hydroxychloroquine should be offered to every high-risk woman rather than only to those who have already had an affected baby [11]. The third is finding a laboratory threshold that means the same thing in every hospital, so that a mother tested in one country can be given the same advice as a mother tested in another [18][19]. None of these will change what happens tomorrow, but each would make the planning that matters most a little more precise.

References

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