X-rays and radiation

“I had an X-ray and did not yet know I was pregnant.” That is one of the questions asked at this practice most often with real fear – and one of the few where the answer is almost always reassuring. This page explains what dose arises from what, what it means, and when X-rays actually come up in gynaecology.

What X-rays are

X-rays are high-energy electromagnetic waves. They pass through tissues to different degrees – bone holds them back more than soft tissue – and the image arises from that difference.

Unlike ultrasound, this is ionising radiation: it can knock electrons out of atoms and thereby alter molecules, including genetic material. The body repairs such damage continuously and reliably; what matters is whether more arises than can be repaired. That is precisely why radiation is not thought of as “harmful or not” but in terms of dose.

The dose effective for the body is measured in millisieverts, abbreviated mSv. For orientation: every person in Austria takes in about 2.5 to 4 mSv a year from natural sources alone – cosmic radiation, radon from the ground, natural radioactivity in food and building materials – depending on where and how they live. This background is the yardstick against which every medical examination can be measured.

What dose arises from what

The range in this table is more than a thousandfold. Anyone treating “X-rays” as one category is comparing a dental image with a CT of the abdomen – two things that have nothing in common beyond the physical principle.

For pregnancy, moreover, what counts is not the dose to the whole body but the dose that actually reaches the uterus. And for anything far away from it, that is vanishingly small: with a dental X-ray or an image of the hand it is practically zero, because the beam runs somewhere else entirely and the rest comes from scattered radiation, which falls off further with a lead apron and distance.

The decisive figure in pregnancy

The specialist societies agree on a threshold: below about 100 mSv at the uterus, no increased risk of malformation is demonstrable. Some sources set the limit lower still, at 50 mSv.

Compare that value with the table above. Practically every single X-ray examination lies two to four orders of magnitude below it. Even a CT of the abdomen – the highest exposure in the list – stays below this threshold.

From that follows the sentence used most often at this practice: an inadvertent X-ray in early pregnancy is practically never a reason to question a pregnancy. This wording is not meant to placate – it corresponds to the recommendations of the international specialist societies.

What remains is a slight, theoretically calculated increase in the risk of cancer in childhood. At the doses named it lies in an order of magnitude that has never been demonstrable in any study in humans, and it disappears into the background probability that exists anyway.

What the timing means

Sensitivity depends on the stage of development, and the sequence is remarkably clearly divided.

The first two weeks after fertilisation – that is, before a missed period: here the “all or nothing principle” applies. Damage in this phase either means that the cell cluster does not develop further and the pregnancy ends unnoticed, or it is fully compensated, because the cells are still interchangeable. Lasting malformations do not arise from it.

Third to eighth week – organ development: this is the most sensitive phase. Only here would malformations be conceivable at very high doses. The threshold of 100 mSv mentioned above refers to precisely this period.

Eighth to fifteenth week – brain development: during this time the central nervous system is most sensitive. The threshold applies here too; below it no effect is described.

From the sixteenth week: sensitivity declines further. What remains is the theoretical cancer risk, which exists across the whole pregnancy.

When an X-ray is necessary

There are situations in which an X-ray in pregnancy is necessary: a broken bone, severe pneumonia, suspected pulmonary embolism, an accident. In all these cases the answer is the same – a necessary examination is carried out.

The reason is a balance that has been unchanged in the radiation protection literature for decades: the risk of an undetected and therefore untreated illness in the mother is orders of magnitude higher than the radiation risk to the baby. A missed pulmonary embolism endangers both; a chest X-ray does not.

Of course everything is done as sparingly as possible: lead apron, the tightest possible collimation of the beam, as few images as possible, and wherever it works, a method without radiation. This principle has its own name – in the radiation protection literature it is called ALARA, “as low as reasonably achievable”. The word “reasonably” carries the weight here: the aim is not the lowest conceivable dose but the lowest that still gives a usable answer.

In any case, say that you are pregnant or might be – not so that the examination is left undone, but so that it is done properly. That distinction matters: saying so should not lead to necessary diagnostics being omitted.

X-rays while breastfeeding

Here the answer can be given in one sentence, and is still rarely believed: after an X-ray, CT or MRI scan you may go on breastfeeding without restriction. No milk has to be expressed and discarded, and no interval has to be observed.

The reason is the same as with the question of waiting before a pregnancy: X-rays leave no residue in the body. Once the machine is switched off, no radiation is present – neither in the tissue nor in the milk.

The iodine-containing contrast media of CT and the gadolinium of MRI also pass into milk only in vanishing amounts and are barely absorbed by the baby. The specialist societies expressly regard breastfeeding afterwards as safe.

There is one genuine exception: nuclear medicine examinations with radioactive substances, such as a thyroid scan. There, waiting times of hours to days apply depending on the substance used, occasionally longer. That is stated specifically by the department carrying it out – ask about it, and express milk during that time so that your supply is maintained.

Ultrasound and MRI: the alternatives

Ultrasound is the first choice in pregnancy and sufficient for many questions – abdominal organs, kidneys, blood vessels, and the baby in any case. It works without radiation. There is more on the page Ultrasound.

Magnetic resonance imaging uses magnetic fields and radio waves, not ionising radiation. It is regarded as safe in pregnancy and is used where ultrasound cannot go further. Two qualifications belong with it: in the first trimester it is used more cautiously, because the data are thinner there, and the contrast agent gadolinium is avoided in pregnancy because it crosses the placenta. An MRI without contrast is not affected by that.

Computed tomography is the routine examination with the highest radiation dose and is used in pregnancy only where clearly necessary – but then without hesitation, see above.

Mammography

Mammography is the only X-ray examination that comes up regularly in gynaecology. It belongs to the Austrian breast cancer screening programme.

The dose of about 0.4 mSv corresponds to roughly seven weeks of natural background. The risk of the examination itself causing a cancer is many times smaller than the benefit of finding an existing one early – in every age group for which the programme applies.

Why registration is needed under 45: before that age breast tissue is often denser. Dense tissue appears white on the X-ray image – as does a tumour. That lowers what the image can tell you, and the number of false positive findings rises. So a deliberate decision stands at the start rather than an automatic invitation.

With dense glandular tissue, mammography is supplemented by ultrasound, which has its strength precisely there. In pregnancy and while breastfeeding, ultrasound is the method of choice anyway, because glandular tissue is then particularly dense. A palpable lump in pregnancy always needs investigating – the idea that everything at this time is hormonal has already delayed diagnoses.

What is due at your age is shown by the screening planner.

X-rays at the dentist

This question comes up so often that it deserves its own section. A single dental image gives about 0.005 mSv – the dose of half a day of natural background. Practically none of it reaches the uterus, because the beam hits the jaw and not the abdomen.

Dental treatment belongs in pregnancy, not afterwards: gum infections demonstrably raise the risk of premature birth. Local anaesthesia is also possible; preparations are used that have been studied in pregnancy. Tell the dental practice, and the choice will be made accordingly.

That the gums become more sensitive in pregnancy and bleed when brushing is hormonal and common. It is not a reason to brush less but to do it more carefully and with a softer brush – and a good occasion for a check-up that is due anyway.

How a mammogram is done

The breast is placed between two plates and compressed, usually in two directions per side. The compression is the most unpleasant part and at the same time the reason the examination works: it spreads the tissue evenly, reduces overlap and considerably lowers the radiation dose needed. It lasts a few seconds per image.

The appointment is best placed in the first half of the cycle, shortly after the bleeding – the tissue is then least tender. Deodorant, powder and creams in the underarm area should be left off, because they appear on the image as small bright spots and can be confused with calcifications.

One feature of the Austrian programme deserves mention: every image is assessed by two specialists independently of one another. This double reading distinguishes a screening programme from a mammogram arranged on your own, and is the reason taking part in the programme is preferable to an individual examination.

Honesty requires the other side as well. A screening programme finds not only cancer, it also produces findings that turn out to be harmless – with weeks of worry in between. And it finds changes that would never have caused symptoms. These disadvantages are real and are named in the consultation; they do not change the fact that the balance in the intended age groups falls in favour of taking part.

Radiation at work and in everyday life

If your work involves radiation – in radiology, in the operating theatre, in nuclear medicine, in dentistry or in research – you should report a pregnancy early. Not because work then stops, but because separate protective rules follow from it. Austrian radiation protection law sets a considerably lower maximum dose for the unborn child than for the employee herself, and the workplace radiation protection officer ensures compliance – usually through a change of activity, not through a ban.

In everyday life, by contrast, the worries are almost always in the wrong place. Air travel raises radiation exposure measurably, but even a long-haul flight is in the range of a chest X-ray; for occasional flying in pregnancy that is not an argument. Airport security scanners work with metal detectors or millimetre waves and emit no ionising radiation. Screens, mobile phones, Wi-Fi and microwaves produce no ionising radiation – physically they belong in a different category and have nothing to do with X-rays.

The only everyday source that really counts is radon from the ground. In parts of Austria exposure is raised; it concerns the lungs over decades, however, and is not a question of pregnancy.

And if it has already happened?

Then, in order: write down which examination took place, when, and in which week. Ask the radiology department about the dose – it is documented and can be estimated for the uterus. And then interpret it together.

In the great majority of cases this conversation ends with reassurance that is not a platitude but a calculation. What remains is the regular care with the examinations provided for anyway – in particular the anomaly scan at 18 to 22 weeks.

If you want to be cautious in advance: examinations of the abdomen and pelvis can be placed, when trying to conceive, in the first ten days after the start of bleeding, when pregnancy is reliably excluded. That rule comes from a time when pregnancy tests were less reliable; today a test is usually enough.

Where to go from here

If you are worried about an X-ray – before, during or after a pregnancy – that is a good reason for an appointment. Bring the details of which examination took place when. Make an appointment.