A Cyst on the Twenty-Week Scan: What Four New Studies Tell Families About Congenital Lung Malformations

Plain-language coverage of four 2025–2026 studies on predicting, watching, and operating on cystic lung lesions found before birth

Being told at a routine pregnancy scan that your baby has a cyst in one lung is frightening, and the honest headline is reassuring: most babies with this finding are born breathing normally and go home with their families. Four studies published between late 2025 and mid 2026 help explain which babies need extra preparation at birth, why doctors increasingly wait rather than rush to operate, and what surgery involves when it is needed. This article walks through what they found and what it means for parents.

What the condition is

The condition is called a congenital pulmonary airway malformation, usually shortened to CPAM. "Congenital" means present from birth, and the rest of the name describes what it is: a part of one lung that formed abnormally while the baby was developing, producing a patch of cystic tissue instead of normal air sacs. It is the most common developmental abnormality of the lower airways, and it is not caused by anything a parent did or did not do during pregnancy [1]. The lesion sits in one lobe — lungs are divided into lobes, three on the right and two on the left — while the rest of the lung develops normally. Because young children grow new lung tissue for several years after birth, losing one lobe does not mean losing a fifth of a lifetime's breathing capacity.

How families used to face this, and what changed

Before detailed pregnancy ultrasound became routine, nobody knew the lesion was there. Families found out in one of two ways: either a newborn struggled to breathe in the first days of life and needed emergency surgery, or a toddler kept getting pneumonia in the same part of the chest until an X-ray revealed the cause. Neither route allowed any planning, and neither gave parents time to ask questions.

Ultrasound changed that, but it also created a new kind of anxiety: knowing about a problem months before anything can be done about it. In 1985, a research team led by Dr N. Scott Adzick described what happens to these lesions during pregnancy when they are watched rather than acted upon, and made a surprising discovery — many of them appear to shrink [2]. They do not usually disappear; rather, the baby's chest grows faster than the lesion does, so the lesion takes up proportionally less space as the pregnancy continues. A minority, though, keep growing and can press hard enough on the heart and developing lungs to make the baby seriously unwell before birth.

Doctors needed a way to tell those two groups apart, and in 2002 a team led by Dr Timothy Crombleholme provided one. They measured the lesion's volume and divided it by the circumference of the baby's head — a measurement that grows predictably — to produce a single number now called the CPAM volume ratio, or CVR. A ratio above 1.6 marked out the babies at real risk of serious complications before birth [3]. That number has guided how closely pregnancies are monitored ever since.

What the number could never settle was what to do after the baby is born well. Some hospitals removed every lesion on the reasoning that it might cause infections or, very rarely, turn into a tumour later in life. Others watched and waited, operating only if the child developed symptoms; specialist reviews still describe this as an open disagreement between centres [4]. Attempts to settle it by pooling earlier studies found that some children who start out well do later develop symptoms, without being able to say which ones [5], and a later pooled analysis suggested planned surgery had fewer complications than waiting — though the children in those two groups were not comparable to begin with, since the biggest lesions were the ones being removed [6].

The first study: the baby matters more than the scan

The largest of the four new studies followed 145 pregnancies diagnosed at three hospitals in Hunan Province, China, between January 2019 and May 2024 [7]. Of the 129 babies born alive, 40 eventually had surgery and 89 did not.

At first glance the fetal measurements looked decisive: babies who ended up having surgery were more likely to have had a high volume ratio before birth, and more likely to have had the heart pushed to one side by the lesion. But when the researchers took account of what the babies were actually like after birth, the picture changed. Once breathing symptoms in infancy were included in the analysis, the fetal measurements no longer predicted surgery on their own. Only breathing symptoms did. In other words, a high fetal measurement does not mean an operation is coming; it means the baby is more likely to develop the breathing symptoms that would lead to one — and many babies with high measurements never develop them.

Two other findings are useful for parents. Nearly all the operations — 38 of the 40 — happened within the first 20 months, at an average age of about seven and a half months, so most families learn fairly early whether surgery will be part of their child's story. And among the 129 children followed for an average of just over two years, none developed a cancer in the lesion. That follow-up is far too short to answer the cancer question properly, and the researchers say so, but it does support watching a well baby rather than operating out of fear.

The second study: knowing what to expect on the day of birth

The second study, at Kanagawa Children's Medical Center in Yokohama, Japan, asked a narrower and very practical question: can the fetal measurement predict which babies will need an operation in their first week of life? The team reviewed 70 babies with cystic lung conditions born between January 2013 and June 2023, of whom nine needed surgery within seven days [8].

Across the whole group, the measurement peaked around 26 weeks of pregnancy and then fell substantially by around 34 weeks — the shrinking effect described decades earlier, now documented with repeated measurements. Parents are often not warned that this decline is normal, and seeing the number fall can be one of the more encouraging parts of a monitored pregnancy.

The babies who needed early surgery had much higher measurements throughout, and were more likely to have excess amniotic fluid or fluid building up in the body before birth. The researchers identified cut-off values for predicting surgery in the first week. What matters most about them is not what a high number predicts — a high number was often followed by no early surgery at all — but what a low number rules out. A baby whose final measurement before delivery was low had roughly a 96% chance of not needing surgery in the first week, which supports delivering in a local hospital and planning to take the baby home.

The third study: a gentler route for babies who do struggle

A small number of babies do have breathing difficulty after birth, and they have historically faced emergency open chest surgery in the first hours or days — a bigger operation, on a less stable baby, than anyone would choose.

Surgeons at the Second Hospital of Hebei Medical University in Shijiazhuang, China, described a different sequence in four babies treated between May 2020 and December 2023 [9]. When a baby's oxygen levels dropped, they used ultrasound to guide a fine drainage tube through the chest wall into the largest cyst, releasing the trapped air. This relieved the pressure and, in all four babies, avoided the need for a breathing machine. That bought time to check the heart — all four turned out to have an additional heart abnormality — and to wait until the baby was stable. Keyhole surgery was then done on days 4 to 29 of life rather than as an emergency, and all four babies did well, with no complications, over six months to four years of follow-up.

Four babies is a very small number, and there was no comparison group, so this is a promising approach rather than a proven one. The idea behind it is easy to follow: a breathing machine pushes air into the cyst and makes it bigger, whereas draining the cyst makes it smaller. Babies with symptoms are known to have a harder course than babies without them [10], so anything that converts an emergency into a planned operation is worth studying properly.

The fourth study: how much lung to remove

The last study, from "Vittore Buzzi" Children's Hospital in Milan, Italy, compared two kinds of operation in 31 children operated on between January 2005 and December 2024 [11]. Twenty-two had the whole affected lobe removed. Nine had a smaller, lung-sparing operation that took out only the abnormal portion.

Removing the whole lobe took about an hour longer. But none of those 22 children needed a second operation, whereas three of the nine who had the smaller operation did. The reason is that this kind of abnormal tissue often extends microscopically beyond what surgeons and scans can see, so a smaller operation can leave a little behind. The trade-off is worth understanding: the smaller operation sounds gentler, but it more often has to be repeated. The researchers also found that children operated through a traditional open incision were more likely to develop a change in chest shape later than those operated by keyhole surgery — a good reason to ask where keyhole surgery is available.

What this means for your family, and what is still unknown

Together these four studies support a calm, staged plan. The fetal measurement, repeated through the pregnancy, is used to decide where your baby should be born and what should be ready on the day. After birth, the decision about surgery is based on your baby — on whether they are breathing comfortably, feeding, and growing — not on a number from before they were born. If your baby does have breathing trouble, draining the cyst first may turn an emergency into a planned operation. And if surgery is needed, removing the whole lobe is more likely to be a one-time event.

Two questions remain genuinely open, and any team that tells you otherwise is going beyond the evidence. Nobody yet knows how children's lung function compares at school age between those who had surgery and those who were watched, because long-term breathing measurements after this surgery are still limited [12]. And nobody can put a reliable number on the lifetime cancer risk of a lesion left in place, because that would require following children for decades. Researchers are working on both; the next step most of them call for is a large study across many hospitals that measures lesions throughout pregnancy and then follows the children into school age. Until that exists, it is entirely reasonable to ask your team what they know, what they are guessing, and why they recommend what they recommend.

References

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  2. Adzick NS, Harrison MR, Glick PL, Golbus MS, Anderson RL, Mahony BS, et al. Fetal cystic adenomatoid malformation: prenatal diagnosis and natural history. J Pediatr Surg. 1985;20(5):483–488. doi:10.1016/S0022-3468(85)80470-X80470-x) ↩
  3. Crombleholme TM, Coleman B, Hedrick H, Liechty K, Howell L, Flake AW, et al. Cystic adenomatoid malformation volume ratio predicts outcome in prenatally diagnosed cystic adenomatoid malformation of the lung. J Pediatr Surg. 2002;37(3):331–338. doi:10.1053/jpsu.2002.30832 ↩
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