MRI in pregnancy: is it safe, and what happens during the scan

MRI uses strong magnetic fields and radio waves rather than X-rays to take detailed pictures of the inside of the body. The NHS says it is used to find out what is causing symptoms, check bones and organs, and see how well a treatment is working. The sources read for this page do not list specific indications in pregnancy; the reasons are decided case by case by the doctors involved.

NHS Lothian's leaflet explains that a scan in pregnancy is arranged when the referring doctor and a radiologist agree it will be of benefit to you.

St George's Hospital NHS leaflet says lots of research has been carried out into whether the magnetic fields and radio waves used in MRI could pose a risk to you or your baby, that no evidence has been found to suggest there is a risk, and that MRI scans are one of the safest medical procedures available during all stages of pregnancy.

The Scottish NHS leaflets use similar wording but frame it in terms of specific effects. They say the risks to the baby are very low, that UK practice follows safety guidelines issued by the Medicines and Healthcare products Regulatory Authority (MHRA), which recommend that scanning of pregnant patients is carried out in a way that minimises risk to the foetus, and that there is no evidence the main magnetic field is harmful at any stage of pregnancy.

Not every hospital leaflet is equally relaxed. A Lancashire Teaching Hospitals liver MRI leaflet asks you to contact the department if you are or think you may be pregnant, noting that MRI is not known to be harmful but is not advisable in pregnancy unless there are special circumstances. That is a reminder that the decision is always individual and made with a clinician.

The Scottish leaflets set out the main theoretical concerns and what is done about each. For heating, they explain that the scanner delivers energy to tissue that can cause warming and that the developing baby is sensitive to heat; to reduce this the team limits the amount of energy by limiting the number of scans, carefully choosing the types of scan and selecting the scanning mode, and the scanner monitors the energy delivered. The scanner uses your weight and height to estimate how much energy can be given. St George's adds that a special setting called Normal Operating Mode is used so that you and the baby do not get too warm and the noises are not too loud.

For noise, you will be given ear plugs or headphones. The leaflets are honest that they cannot protect the baby's ears directly, but say your body and the fluid sac around the baby reduce the noise reaching the baby, and that scans may be chosen to reduce the noise level.

Some MRI scans use an injection of a contrast agent, a type of dye that helps tissues show up. NHS Lothian explains that gadolinium-based contrast agents are used in around 1 in 3 MRI scans, that there is very little information on their use in pregnant women, and that the available studies give different results, so safety in pregnancy remains uncertain. It says the Royal College of Radiologists advises that contrast agents are only given to pregnant patients when it is absolutely necessary, and quotes the College as stating that no effect on the developing foetus is anticipated.

In practice this means that a radiologist reviews the request, and the scan is done without contrast wherever the question can be answered that way. If contrast is needed, you should be told why, and you can ask questions before agreeing. The NHS MRI page adds that you will be asked on the safety form whether you are pregnant, breastfeeding or have kidney problems, and about allergies including to contrast.

The St George's leaflet describes a standard routine. You attend in comfortable clothing without metal zips or buttons if possible, remove jewellery and watches, and complete an MRI safety questionnaire about surgery or implants, which a radiographer goes through with you. You lie on a table that moves into the scanner, head first or feet first depending on the body part, with equipment placed over the area being imaged. You need to keep as still as possible, are given a buzzer to speak to the radiographer, and the scan lasts between 15 and 90 minutes depending on the area. Afterwards you can go home, eat and drink as normal and carry on as usual.

The NHS adds that a scan usually takes between 15 minutes and 1 hour but can take longer, that you may be asked to hold your breath for parts of it, and that results usually take 1 to 2 weeks to reach the doctor who referred you. Ask the team how and when you will be told the result.

If you are claustrophobic, say so in advance. St George's notes that going in feet first may be easier, although it is not always possible. You are given a buzzer, so you can alert the radiographer at any time if you feel unwell.

• Tell the department you are pregnant, or might be, before the appointment.

• Mention any implants, devices, previous surgery and allergies on the safety form.

• Ask whether contrast will be used and why.

• Wear comfortable clothing without metal.

• Ask how and when you will receive results.

Other imaging tests, such as ultrasound, may be considered first or instead. Which test is right depends on the clinical question, and your obstetric team and a radiologist make that judgement; if you are unsure why MRI was chosen, ask them.

In the UK, scans in pregnancy are requested through the NHS maternity or hospital team and reported by NHS radiologists. Private MRI services are also available in the UK. The UK safety guidance cited in the NHS leaflets applies to MRI generally, and a clear clinical reason for scanning a pregnant person is still needed.