The Weighting Game

David Lloyd_crop

One of the most important things to get right when imaging an unborn baby is the fetal weight. How do we know the fetus is growing as it should be unless we know how big they are? How can we know that the heart, or brain, or lungs, are developing normally, unless we can compare them to the rest of the body?

Unfortunately, it isn’t so easy as just popping the baby on a set of scales. They are floating in water, attached to a placenta via a long umbilical cord, surrounded by a muscular womb, and – oh yeah! – their mother. So any time we guess how much the fetus weighs, it is just that: a guess.

The most commonly used formula to estimate the fetal weight was developed in the 1980s by Dr Francis Hadlock, using ultrasound to measure the head, abdomen and thighs, and guessing the weight of the baby from that. This method can actually be pretty inaccurate – for example, we know that ultrasound can be a bit blurry, doesn’t define the edge of bones very well, and depends on finding exactly the right angles to measure which might not always be possible. Amazingly though, we’ve not been able to find a better way since then; almost every routine scan in the UK will use this method. As my fetal medicine colleague Jackie Matthew put it: “a lot of people think it’s just down to how good the sonographer is – but it’s really not that simple”. Now though, as part of the iFind project, we are working on new ways of estimating the fetal weight which we hope will be far more accurate.

Zhang & Davidson’s 3D rendering of a fetal MRI

When each of our iFind 2 volunteers attends for an extra ultrasound and MRI scan, we use these to build a three-dimensional “atlas” of the fetus, which will form the foundation for the technologies we develop to screen for fetal abnormalities. Being able to see the baby in “3D” like this is one of the jobs of my colleagues Tong Zhang and Alice Davidson – the latter of whom produced this beautiful rendering of a fetus from an MRI scan. But this image doesn’t just look amazing: it also means we could have a much more accurate way of guessing the baby’s weight than a few blurry ultrasound measurements. Knowing how much space the baby takes up in three dimensions – the fetal volume – means we could potentially estimate far more precisely whether the baby is growing normally.

So that’s a win right? Go iFind! Well… not quite. Unfortunately it’s still not that simple – and that’s where Jackie comes in. Her research has some difficult questions to answer: exactly how inaccurate are these ultrasound techniques? How do we do know? Is MRI really better? How do we prove it? And how does a fetal volume equate to a fetal weight? Is it the same through all nine months of pregnancy?

These are tough questions, but like everyone else at iFind, Jackie is determined to find answers. And when she does, it’s these types of new discoveries that should help iFind get closer to its ultimate goal: using new technologies to improve how we see and understand life before birth.


Dr David Lloyd is a Clinical Research Fellow at King’s College London and working as part of the iFIND project. The overall aim of the intelligent Fetal Imaging and Diagnosis project is to combine innovative technologies into a clinical ultrasound system that will lead to a radical change in the way fetal screening is performed.


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