When a Baby's Heart Races Before Birth

What three recent studies from Italy, China and the United Kingdom tell families about treating a fast fetal heartbeat and what happens afterwards

Sometimes a routine pregnancy scan finds that the baby's heart is beating far too fast — twice the normal rate or more. Doctors can treat this before birth by giving the mother medicine that crosses the placenta and reaches the baby. Three studies published in 2025 and 2026 help answer the questions parents ask first: which medicine works best, how safe is it, and will my baby still have this problem after birth?

Why a fast fetal heartbeat matters, and how families used to face it

A baby's heart normally beats between about 110 and 160 times a minute in the womb. Irregular beats are picked up in roughly one or two pregnancies in a hundred, and almost all of them are harmless extra beats that settle on their own. A much smaller number of babies develop a sustained fast rhythm — a heart rate above 180 beats a minute that does not stop. This is called supraventricular tachycardia, or SVT, and it matters because a heart racing continuously for days cannot fill properly between beats. Over time the baby can develop heart failure and then a condition called hydrops, in which fluid collects in the baby's chest, abdomen and skin. Before effective treatment existed, some of these babies died before birth.

For decades, the response to this was largely improvised. Doctors knew from small studies published in the early 2000s that different types of fast rhythm behaved differently, and that treating the mother could often slow the baby's heart [1]. One medicine, digoxin — a very old heart drug derived from the foxglove plant — became the standard first choice in most hospitals, and a major American Heart Association guidance document in 2014 reflected that practice [2]. But this convention was never tested the way modern medicines are tested. There has never been a study in which mothers were randomly assigned to one drug or another, which is the only way to know for certain which works better.

Evidence did accumulate, piece by piece. A study across several hospitals compared digoxin with two other drugs, flecainide and sotalol, and found that which drug worked best depended on the exact type of fast rhythm and on how unwell the baby already was [3]. Two studies in 2016 argued that flecainide was the stronger choice [4][5], and a review pooling all the available research came to a similar conclusion [6]. Sotalol built its own case, particularly for a rhythm called atrial flutter [7]. A carefully planned study across hospitals in Japan tested a step-by-step treatment plan [8] and later followed the children to age three, finding their development was broadly typical [9]. Even so, the most recent American Heart Association statement in 2024 still describes the choice of first drug as based on expert agreement rather than firm proof [10].

The three new studies

The first study comes from Monaldi Hospital in Naples, the specialist referral centre for southern Italy [11]. Doctors there reviewed every baby diagnosed with a sustained fast rhythm over ten years — 32 pregnancies between January 2014 and December 2024. Their approach was to use detailed ultrasound of the baby's heart to work out exactly which type of fast rhythm was present, and then choose the medicine to match.

The second comes from West China Second Hospital in Chengdu, and it took the opposite approach [12]. Between January 2009 and February 2022, 32 babies with a sustained fast rhythm were all given digoxin first, no matter what type of rhythm they had. Because every family followed the same plan, this study shows what digoxin alone actually achieves — something a study where doctors pick and choose cannot show.

The third, from Evelina London Children's Hospital, asked a different question altogether [13]. Fifteen pregnant women wore a small monitor with sticky pads on the abdomen overnight at home, recording the baby's heartbeat electrically for hours at a time — something like the heart monitors adults wear, but listening through the mother's abdominal wall.

What they found

In Naples, treatment worked well. Of 27 babies treated, 20 (about three in four) returned completely to a normal rhythm, and another 6 improved substantially [11]. One baby died before birth despite treatment, from a rhythm that would not stop combined with severe fluid build-up. Among the babies who already had heart failure or hydrops when treatment started, 10 of 11 recovered — a striking result, because these are the babies families are told to worry most about.

The comparison between drugs is worth quoting carefully. All 11 babies treated with flecainide alone responded, 10 of them completely. Of the 11 treated with digoxin alone, 7 responded, 6 completely — meaning 4 did not respond at all. The Italian doctors are honest that with numbers this small, the difference could have arisen by chance, and they say so explicitly.

The Chengdu study is where the digoxin numbers become hard to ignore. When every baby received digoxin first, fewer than half — 14 of 32 — returned to a normal rhythm within a week, and 17 of 32 did not respond at all [12]. The researchers also found two useful warning signs. Babies whose heart valves were leaking were far less likely to respond to digoxin. And among babies who did respond, those whose heart function had not improved by the first follow-up scan were more likely to relapse — which usually happened within a few days of the mother's dose being reduced.

The London study showed something clinicians have long wanted. Recordings lasted about eight and a half hours on average, and useful information was obtained from 22 of 23 recordings [13]. The traces were clear enough to tell different types of fast rhythm apart and matched what the hospital ultrasound had shown. In two pregnancies, recordings made before and after the mother started medicine documented the baby's heart settling back to normal. In one baby whose average heart rate looked entirely normal, the recording caught brief bursts of racing lasting only four to ten beats — episodes no clinic scan would ever have found. Only the direct method that fetal cardiologists consider most accurate, a magnetic scanning technique, can read the baby's electrical signal more precisely, and it exists in only a handful of centres worldwide [14].

What this means for a family going through it

The most reassuring finding for parents is about what happens after birth. In Naples, 10 of 26 babies — nearly four in ten — had no arrhythmia at all after delivery and needed no medicine, confirmed by careful electrical testing done off treatment [11]. Babies diagnosed earlier in pregnancy were more likely to be in this group. And importantly, having had heart failure or hydrops before birth did not mean the rhythm problem was more likely to persist afterwards — a finding that matches a 30-year experience from another specialist centre [15]. Being the sickest baby before birth does not mean being the sickest baby afterwards.

The Chengdu team followed their children for anywhere from two months to nearly thirteen years [12]. Growth was normal in all the children assessed, quality-of-life scores were high, and among eleven children screened with a standard developmental questionnaire, only three had a single area of mild concern. This is a small group and not a substitute for larger studies, but it is genuinely encouraging.

The medicines themselves need respect rather than fear. In Naples, 4 of 27 mothers (about one in seven) had a side effect — one felt weak, and three had minor changes on their heart tracing [11]. Every one was managed by lowering the dose. Mothers had a heart tracing before starting and at every visit, and blood levels were checked for digoxin. The Chengdu team recorded no side effects at all. The picture is of drugs that require monitoring but are, with that monitoring, well tolerated.

What researchers are working on next

The single biggest gap is the one that has been open for fifty years: no one has ever run a study that randomly assigns mothers to flecainide or digoxin as the first drug. Everything we know comes from comparing groups that doctors chose, which always leaves room for doubt — a point made directly in a commentary published alongside the Italian study [16]. Designing such a trial has been difficult partly because this condition is rare and partly because there was no good way to measure how much of the day a baby actually spends in a fast rhythm.

That second obstacle may be lifting. If a monitor worn at home can measure exactly how much time a baby spends with a racing heart, and whether that time falls after treatment starts, researchers finally have a way to compare two drugs fairly. It would also mean families living far from a specialist centre could be monitored without repeated long journeys, and that hospitals in countries without fetal cardiology services might one day be able to help these babies too.

For now, the practical message is a hopeful one. A fast fetal heartbeat is serious, it is treatable, most babies respond, and a substantial number arrive with the problem already behind them.

References

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