Fetal MRI (Magnetic Resonance Imaging)

MRI (Magnetic Resonance Imaging) creates images using a powerful magnetic field and radio waves. A computer turns the signals from the water in your baby's tissues into highly detailed pictures. Two points are important for parents:

• No ionising radiation: Unlike X-rays or CT scans, MRI uses no radiation, and it is considered safe in pregnancy after the first trimester.
• An adjunct, not a replacement: Ultrasound remains the primary tool for pregnancy imaging—it is real-time, widely available, and excellent. Fetal MRI adds information in specific situations, particularly where soft-tissue detail (especially of the brain) or a wider field of view is needed. The two tests work together.

No contrast dye is given for a routine fetal MRI, and the scan does not touch or enter the body.

Fetal MRI is offered for particular clinical questions rather than as a routine test. The main indications include:

• Brain and central nervous system: This is the most common reason. MRI adds detail in ventriculomegaly (enlarged fluid spaces), problems of the corpus callosum (the band connecting the two halves of the brain), abnormalities of the posterior fossa (the back of the brain, including the cerebellum), and subtle disorders of brain development.
• Congenital diaphragmatic hernia (CDH): MRI measures lung volume and clarifies whether the liver is in the chest, which helps predict severity and plan care.
• Neck masses and airway: MRI maps large neck masses and assesses whether the airway may be at risk at birth, informing delivery planning.
• Complex anomalies before fetal surgery: Detailed anatomy from MRI helps the surgical team plan procedures such as open or fetoscopic spina bifida repair.
• Where ultrasound is limited: for example by maternal body habitus, low fluid, or the baby's position.

Fetal MRI is usually performed from around 18–20 weeks onwards, because the baby is large enough for the anatomy to be clearly resolved; brain questions are often imaged a little later. What to expect:

1. Preparation: No special preparation, no dye, and no injection. You may be asked about anything metal in your body, as with any MRI.
2. During the scan: You lie on a couch that moves into the scanner. You can usually lie on your back or slightly on your side for comfort. The machine is noisy—you will be given ear protection—and you must stay still while short sequences are taken.
3. Duration: Typically 30–45 minutes. Fast imaging sequences are used to capture the baby despite fetal movement.
4. No sedation: Neither mother nor baby is sedated for a standard fetal MRI.
5. Reporting: The images are reviewed by specialists in fetal imaging and correlated with your ultrasound findings before a combined conclusion is reached.

The value of fetal MRI lies in how it refines diagnosis and therefore your care:

• Confirming or refining a diagnosis: MRI may confirm the ultrasound finding, add important detail, or occasionally identify additional findings that change the picture.
• Predicting severity: In CDH, lung-volume measurements help estimate how the baby's breathing may be affected after birth, guiding whether fetal therapy might be considered and how the neonatal team prepares.
• Planning delivery and surgery: Mapping a neck mass or complex anomaly informs where and how you should deliver and whether specialist teams need to be present.
• Improving counselling: A clearer diagnosis allows us to give you more accurate information about what to expect, supporting your decisions.
• Working with the wider team: Findings are discussed in a multidisciplinary setting—fetal medicine, radiology, neonatology, and surgery as needed—so that a coordinated plan is made.

Fetal MRI is a safe, well-established test with a strong record when used appropriately:

• Safety: With no ionising radiation and no contrast, fetal MRI is regarded as safe after the first trimester. Large experience over decades supports this.
• Complementary strength: In experienced hands, MRI adds clinically useful information beyond ultrasound in a meaningful proportion of the cases in which it is requested—especially for the fetal brain.
• Not a screening test: Fetal MRI does not replace your anomaly scan and is not used to 'check everything'. It answers a specific question raised by ultrasound.
• Limitations: Image quality can be affected by fetal movement, and MRI does not assess blood flow the way Doppler ultrasound does. This is why the two tests are always interpreted together.

Fetal MRI is arranged by your maternal-fetal medicine team when a specific question needs answering—you would not usually request it yourself. It is typically considered when:

• An ultrasound has found a brain, chest, neck, or complex abnormality that would benefit from additional soft-tissue detail.
• Fetal surgery is being planned and precise anatomy is required.
• Ultrasound views are limited and an alternative image would help.

Our MFM and fetal-imaging team decides with you when fetal MRI will add value, arranges the scan, and integrates the results with ultrasound to build a coordinated plan aligned with recognised fetal-imaging practice.