When a Baby Grows Too Slowly Before Birth: What Doctors Can Now Predict, and What They Still Cannot

A plain-language look at four 2025–2026 studies on monitoring, risk estimates and brain development in pregnancies affected by poor fetal growth

Some babies grow too slowly before birth because the placenta — the organ that carries oxygen and nourishment from mother to baby — is not working properly. Doctors cannot repair a failing placenta, so the only real decision left is when to deliver. Four studies published in 2025 and 2026 examine how well that decision is now being made, and how much can honestly be said to parents about their baby's future.

Why some babies grow too slowly

Throughout pregnancy the placenta acts as lungs, gut and kidney all at once. When it works poorly, the fetus receives less oxygen and fewer nutrients than it needs, and it responds the way any body under shortage responds: it slows down. Growth tails off first, and blood flow is redirected towards the organs that matter most for immediate survival — heart, adrenal glands and, above all, the brain. Doctors can see this redirection on an ultrasound scan by measuring how blood moves through the umbilical cord and through a large artery in the baby's head.

This condition is called fetal growth restriction, and when it appears before 32 weeks of pregnancy it is described as early-onset. It is not the same as simply being a small baby: some babies are small because their parents are small, and they are perfectly healthy. Distinguishing the two mattered so much, and was done so inconsistently between hospitals, that in 2016 a large group of specialists agreed on a formal definition combining the baby's measurements with the blood-flow findings [1]. Only after that agreement could studies from different countries be sensibly compared. (For how doctors judge whether any newborn is small for its age, see In[Neo]Sight's article on rebuilding US birth-size charts.)

What families used to face

For decades, parents in this situation were given very little that was specific. Doctors knew the pregnancy was dangerous, but they did not agree on which warning sign should trigger delivery, and the trade-off was brutal in both directions. Deliver too early and the baby faces the full weight of extreme prematurity — immature lungs, a fragile brain, weeks or months in intensive care. Wait too long and the baby may be harmed by lack of oxygen, or may die before birth. Different hospitals used different rules, and families often received conflicting advice.

That began to change with a study conducted across several European countries, the Trial of Randomized Umbilical and Fetal Flow in Europe — known by its short name, TRUFFLE — which compared two different signals for deciding when to deliver: a computer measurement of subtle changes in the baby's heart-rate pattern, and a change in blood flow through a small vessel in the baby's liver called the ductus venosus [2]. Babies monitored using the liver-vessel signal did somewhat better at two years of age. But when the researchers looked closely, they found that most of those babies had actually been delivered because of the heart-rate measurement anyway. The lesson was that both signals are needed, and international guidelines now recommend using them together [3].

Meanwhile, attempts to treat the problem itself have all failed. A large Dutch trial found that inducing labour near term worked no better and no worse than watchful waiting [4], and a British trial of sildenafil — a drug hoped to widen the blood vessels supplying the womb — showed no benefit at all [5]. There is still no medicine that makes a failing placenta work better. This is why so much research effort has moved to a different question: not how to fix it, but how accurately we can predict what will happen.

Does careful monitoring work in ordinary hospitals?

A trial result is one thing; everyday practice is another. Trials are staffed generously, follow their rules strictly and enrol carefully chosen patients. So a fetal medicine unit in London — one of the hospitals that had taken part in the original TRUFFLE trial — looked back at eleven and a half years of ordinary care, from January 2013 to July 2024, in 171 pregnancies where the baby was growing poorly before 32 weeks [6].

The results were close to the trial's. Babies were born at a median of 30 weeks and 3 days, weighing around 868 grams — a little under two pounds. Five babies were stillborn (2.9%) and ten died after birth (5.8%), so 155 of the 171 (90.6%) survived without signs of brain injury. Only one baby in the whole group had the type of oxygen-related brain injury that doctors most fear.

Two findings from this study are especially useful for families. The first is about time. On average, 17 days passed between the diagnosis and the birth — but when the problem was picked up before 26 weeks, that interval stretched to 31 days, roughly three times longer than for pregnancies diagnosed later. Parents told very early that their baby is not growing often assume delivery is days away; frequently, it is not. The second is that the reason for delivering changed as pregnancy advanced. Before 32 weeks, most deliveries were prompted by the heart-rate measurement or the liver-vessel signal. After 32 weeks, most were prompted by the cord blood flow or by the mother's own health, particularly pre-eclampsia, a blood-pressure complication of pregnancy.

Putting a number on the risk

The second study tackles the question parents ask on day one: what are the chances? Researchers used records from 1,453 pregnancies managed in six large Dutch hospitals between 2012 and 2021 to build a calculator that estimates risk at the moment the mother is admitted for monitoring, rather than in the last hours before birth [7].

The calculator uses fourteen pieces of information that are already available on the day of admission — the baby's estimated weight, how many weeks pregnant the mother is, whether she has pre-eclampsia, the blood-flow measurements, and several details of her medical history. Tested against the data it was built from, it correctly ranked which pregnancies would run into trouble about 83% of the time, which is good for this kind of tool.

But what counts as "trouble" needs unpacking, because the phrase hides a great deal. The researchers used a bundle of problems grouped together: death, a serious bowel infection, chronic lung disease of prematurity, two kinds of brain injury, or a blood infection. That bundle occurred in 38.5% of pregnancies — a frightening-sounding number until you see what it is made of. Chronic lung disease (about 21% of babies) and blood infection (about 18%) accounted for most of it. Death in hospital occurred in 7.3%. The most severe form of brain white-matter injury occurred in 0.7% — seven babies in a thousand. Most of what the number measures are the well-recognised complications of being born very early [8], not brain damage. A family told "a 38% risk" without that explanation will hear something far worse than what the doctors mean.

Two honest limitations should be mentioned. The calculator has not yet been tested in hospitals outside the Netherlands, so its accuracy elsewhere is unknown. And it was built only from pregnancies where parents had already chosen active treatment, so it cannot answer the separate and much harder question of whether to intervene at all.

The question everyone asks: what about the brain?

Almost every parent asks this first, and it is where the honest answer is least satisfying. An Italian study of 83 pregnancies looked at whether the blood-flow measurements made before birth could predict problems found in the baby's brain afterwards [9]. They could, but only modestly — the measurements were no better at this than simply knowing how premature the baby was, and became useful mainly when combined with other information. It is also important to know what was being predicted: most of the "brain findings" were minor abnormalities seen on routine head ultrasound scans, whose long-term meaning is genuinely uncertain. Serious bleeding into the brain occurred in about 5% of babies and the most severe form of white-matter injury in none.

What about scanning the baby's brain after birth with magnetic resonance imaging, which produces far more detailed pictures? A 2025 review searched four large medical databases for studies that had done exactly this and then followed the children to see how they developed [10]. Out of nearly 1,700 papers, only three met the standard, they could not be combined statistically, and only one found a link between a newborn brain measurement and later development specifically in growth-restricted babies. All three studies looked only at premature babies, and none followed children beyond two years of age. That last point matters, because the learning, attention and behaviour differences most often reported after poor fetal growth tend to become visible at school age rather than in toddlerhood [11], [12].

So the truthful summary is this: a detailed brain scan in the newborn period cannot currently tell an individual family what their child's development will be like. A reassuring scan is good news, but it is not a guarantee, and the way to answer the developmental question is careful follow-up over years, not a single picture.

What researchers are working on next

Three directions are already clear. The Dutch team wants to test their calculator in other countries and, more ambitiously, to build a version that updates daily rather than giving one estimate at admission — closer to how decisions are really made. The London team argues for more frequent heart-rate monitoring, possibly using devices that let it be done at home, since the changes they rely on can develop within a day. And the imaging researchers are calling for studies large enough to analyse growth-restricted babies as a group in their own right, including babies born at or near full term, followed to school age. Underlying all of this is a shift in how the redirection of blood towards the brain is understood: once described reassuringly as "brain sparing", it is now recognised as a sign of a fetus under strain, and a brain that is protected in this way can still be affected [13], [14].

What this means for your family

If you are being monitored for poor fetal growth, three things from these studies are worth holding onto. Careful monitoring works: in a large ordinary hospital following the recommended plan, more than nine in ten babies survived without signs of brain injury. Time is often longer than you expect, particularly if the problem was found very early. And when you are given a percentage, it is entirely reasonable to ask what it is made of — which problems it counts, how many of them are the ordinary complications of prematurity, and how many concern the brain. The teams caring for you cannot yet tell you how your child will develop. They can tell you why they are watching, what would make them act, and what each number does and does not mean, and it is fair to ask them for all three.

References

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  2. Lees CC, Marlow N, van Wassenaer-Leemhuis A, et al. 2 year neurodevelopmental and intermediate perinatal outcomes in infants with very preterm fetal growth restriction (TRUFFLE): a randomised trial. Lancet. 2015;385(9983):2162–2172. doi:10.1016/S0140-6736(14)62049-362049-3) ↩
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