The Measurement That Decides Whether a Baby Has Surgery Before Birth

What three 2025 studies found about how doctors measure a baby's lungs in the womb, where the line for "severe" should be drawn, and what the fetal operation really involves

Congenital diaphragmatic hernia is a birth defect in which a gap in the muscle below the lungs lets organs from the abdomen move up into the chest, leaving the lungs too small to work well at birth. Some babies can be helped by an operation performed before they are born, and whether a family is offered it depends almost entirely on one measurement of lung size. Three studies published in 2025 examined that measurement — and found it is less exact than the decision resting on it.

Why lung size is the number that matters

The diaphragm is the sheet of muscle that separates the chest from the abdomen and does most of the work of breathing. In this condition it does not close completely during early pregnancy, and loops of bowel — sometimes the stomach and liver too — slide up through the gap and take space the lungs need in order to grow. The result is not a hole that can simply be patched: it is a pair of lungs that have had less room than they needed for months, with blood vessels inside them that are stiff and reactive. Repairing the diaphragm after birth is straightforward surgery. Getting small, stiff lungs to support a newborn is the hard part.

Because the problem develops over the whole pregnancy, doctors have long wanted a way to intervene during it. Since the early 2000s there has been one: a fetal operation called tracheal occlusion, in which a surgeon passes a very fine telescope into the womb and places a small balloon in the baby's windpipe [1]. Fluid that the lungs normally make and release is trapped behind the balloon, and the resulting pressure stretches the lungs and encourages them to grow. The balloon is removed, usually around 34 weeks, so the baby can breathe at birth.

Not every baby with this condition is offered the operation, because it is not free of risk. So doctors need a way to decide who is sick enough to justify it. The measurement they use compares the size of the lung that has more room — the one on the opposite side from the hernia — against the size expected for that stage of pregnancy, and expresses it as a percentage. This measurement was shown in 2007 to predict which babies survive [2], and it has been the basis for decisions ever since. In some centres it is checked with a magnetic resonance imaging (MRI) scan, which measures lung volume in three dimensions.

How the cut-off was decided, and why it has been questioned

A large international research effort called the Tracheal Occlusion To Accelerate Lung growth (TOTAL) trials, reported in 2021, tested the operation properly by randomly assigning families at experienced fetal-surgery centres in several countries either to have the balloon placed or to be watched closely instead. For babies with the most severe lung underdevelopment, the operation made a real difference: 40 out of every 100 survived to go home, compared with 15 out of 100 who were watched [3]. For babies in the moderate group, survival was 63 out of 100 with the operation and 50 out of 100 without — a difference that was not large enough to be certain it was real [4]. A later combined analysis of both trials suggested the moderate group may have done less well because their balloons were placed later in pregnancy, not because their disease was different [5]. European centres later showed the operation also helps babies whose hernia is on the right side [6].

Those trials turned a percentage into a rule. Below one number, the operation is offered; above it, it is not. What nobody had tested at scale was whether two experienced doctors looking at the same baby would arrive at the same percentage.

The first study: the same scans, fourteen different readers

Researchers working through the North American Fetal Therapy Network sent fifteen anonymous MRI scans of babies with this condition to radiologists at sixteen member hospitals across Canada and the United States, and asked each of them to measure the lungs and the position of the liver [7]. Fourteen doctors at fourteen hospitals completed the task. None knew how the pregnancies had turned out.

The results were not chaotic. For total lung volume, the readers were reasonably consistent, whichever of the two standard reference formulas they used [8], [9]. But "reasonably consistent" is not the same as "consistent enough to draw a line through." When the researchers applied the usual cut-off for calling a case severe, twelve of the fifteen babies would have been labelled severe by some readers and not severe by others. Using a stricter cut-off, eight of fifteen still varied. Judging how much of the liver had moved into the chest was the least reliable measurement of all. Even simply saying whether the liver was up or down produced complete agreement in only five of fifteen cases, although the majority answer was correct in fourteen.

Just as importantly, hospitals that handle many cases did no better than those that handle few. That suggests the problem is not individual skill but the absence of a single agreed method for taking the scan and doing the measurement — something this same research network had already pointed out years earlier [10].

The second study: where should the line actually be?

A large American fetal centre looked at this from the other direction. Instead of asking how consistently the measurement is taken, it asked what the measurement predicts. Reviewing 340 pregnancies looked after between 2013 and 2023 — deliberately leaving out any baby who had the fetal operation — the team calculated which values best separated the babies who survived from those who did not [11].

Their answer was that the line should sit slightly higher than where it currently sits: babies below about 28% were noticeably more likely to die and below about 33% were more likely to need a heart–lung bypass machine after birth. But the separation was a gradient, not a cliff. Among babies above their cut-off, 88 in 100 survived; among those below it, 67 in 100 still did. No single number identifies with confidence which baby will die.

There is a further point in that study worth understanding, because it changes what the operation is for. Survival at that centre without any fetal surgery was far better than the 15 in 100 seen in the trial's comparison group. Where survival is already good, the fetal operation may not add much to the chance of living — but it may reduce the amount of intensive treatment a baby needs, including time on a heart–lung bypass machine. Other centres have reported the same pattern [12], [13]. That is still a worthwhile goal, but it is a different conversation from "this operation will save your baby's life."

The third study: what families actually go through

The third paper described what happened to 47 pregnancies treated at Canada's national referral centre for this operation between 2010 and 2024 [14]. The balloon was placed successfully in 47 of 48 attempts, and there were no serious complications for the mothers and no babies lost in the womb — a reassuring safety record across 76 separate procedures.

The costs came elsewhere. In 7 out of 10 pregnancies the waters broke early, and babies were born on average around 36 weeks rather than at term. Half of the balloon removals had to be done sooner than planned, and about a quarter of babies had to be delivered by a special procedure in which the balloon is taken out while the baby is still attached to the placenta. Between 33 and 64 out of every 100 babies survived to go home, depending on how severe their condition was and which side the hernia was on. Of those who went home, about a third still needed extra oxygen and two-thirds were still fed through a tube.

Families also have to stay near the fetal centre for the whole time the balloon is in place — 1 in 5 of the Canadian families had travelled from another province — and the centre has to keep a specialist team available around the clock, because a balloon still in place when a baby is born is a genuine emergency. Where that expertise exists, balloon removal is almost always successful; where it does not, it has cost lives [15]. What happens after birth matters at least as much: the availability of a heart–lung bypass machine turns out to be one of the strongest predictors of survival after this operation [16].

What this means for families, and what comes next

If your baby has been diagnosed with this condition, the most useful thing to take from these three studies is not alarm but a set of fair questions. It is reasonable to ask which measurement your centre used and whether the ultrasound and the MRI agreed. It is reasonable to ask whether the number would place your baby near a cut-off, because near a cut-off is exactly where reader-to-reader differences matter most. It is reasonable to ask whether the operation is being offered in the hope of improving survival or of reducing the intensity of treatment after birth. And it is reasonable to ask about the practical realities — how long you would need to stay nearby, and what the plan is if labour starts early.

Researchers are working on all three of the weaknesses these studies exposed. There are active efforts to agree a single standard way of taking and measuring the scans, so that a percentage means the same thing in every hospital. Larger studies, including one comparing 180 treated with 180 untreated pregnancies in Latin America, are sharpening the estimate of who benefits [17]. And a new type of balloon that deflates by itself when the mother is placed near an MRI scanner was tested in people for the first time in 2026 [18]; if it works reliably, it would remove the second operation and the emergency-delivery problem altogether. Meanwhile, follow-up studies of nearly 400 children treated this way are broadly encouraging: breathing problems are the most common difficulty but tend to improve with time, feeding and reflux problems are frequent, and development is generally reassuring [19]. The measurement that opens the door to this operation is being improved — and knowing that it is imperfect is what makes it possible to ask better questions about it.

References

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