Am Fam Physician. 1999;59(7):1813-1818
See related patient information handout on radiographic imaging in pregnancy, written by the authors of this article.
Maternal illness during pregnancy is not uncommon and sometimes requires radiographic imaging for proper diagnosis and treatment. The patient and her physician may be concerned about potential harm to the fetus from radiation exposure. In reality, however, the risks to the developing fetus are quite small. The accepted cumulative dose of ionizing radiation during pregnancy is 5 rad, and no single diagnostic study exceeds this maximum. For example, the amount of exposure to the fetus from a two-view chest x-ray of the mother is only 0.00007 rad. The most sensitive time period for central nervous system teratogenesis is between 10 and 17 weeks of gestation. Nonurgent radiologic testing should be avoided during this time. Rare consequences of prenatal radiation exposure include a slight increase in the incidence of childhood leukemia and, possibly, a very small change in the frequency of genetic mutations. Such exposure is not an indication for pregnancy termination. Appropriate counseling of patients before radiologic studies are performed is critical.
Many women become ill while pregnant and require acute medical care, including radiographic imaging with ionizing radiation. Exposure of a fetus to radiation can be alarming to parents and is dealt with by the general public with less objectivity than is evident with exposure to almost any other agent.1 Even physicians are at times known to approach this topic in a biased and unscientific manner, leading to poor patient care and inappropriate advice.2 With x-ray usage rates exceeding an average of more than one study for every person in the United States each year,3 it is important for primary care doctors to have a clear perception of the actual risks and benefits of radiographic studies during pregnancy.
Because some studies will be performed before a pregnancy is recognized, even doctors not routinely providing prenatal care should understand these issues. Family physicians must be ready to counsel expectant mothers requiring radiographic imaging and women who have already been exposed. They should also have a firm rationale for ordering such studies when interacting with other clinicians.
A patient at 19 weeks of gestation presented with flank pain and microscopic hematuria. She was diagnosed with pyelonephritis and treated with parenteral antibiotics. Her flank pain progressed despite antibiotic treatment, necessitating a renal ultrasound examination, which was inconclusive. An intravenous pyelogram (IVP) was ordered, but the radiologist refused to perform the study because of concern about radiation exposure to the fetus. Despite further discussion, the study was denied until a perinatologist verified the appropriateness and relative safety of the study.
The IVP revealed two stones, and the patient eventually required ureteral stent placement. Despite treatment, she had progressive renal disease with obstruction, requiring induction of labor at 35 weeks of gestation. At birth, her infant was healthy and weighed an age-appropriate 2,500 g (5 lb, 8 oz).
Understanding the Nature of Radiation
Ionizing radiation (x-ray) is composed of high-energy photons that are capable of damaging DNA and generating caustic free radicals.3 A patient's dose of photons is measured in the gray (Gy) and the rem, or in the older and more commonly recognized unit, the rad. The range of doses provided by common radiographs is outlined in Table 1.4–8
|Examination type||Estimated fetal dose per examination (rad)*||Number of examinations required for a cumulative 5-rad dose†|
|Upper or lower extremity4||0.001||5,000|
|Chest (two views)6||0.00007||71,429|
|Abdominal (multiple views)6||0.245||20|
|Hip (single view)6||0.213||23|
|CT scans (slice thickness: 10 mm)|
|Head (10 slices)6||< 0.050||> 100|
|Chest (10 slices)6||< 0.100||> 50|
|Abdomen (10 slices)6||2.600||1|
|Lumbar spine (5 slices)6||3.500||1|
|Pelvimetry (1 slice with scout film)6||0.250||20|
|Upper GI series6||0.056||89|
|Nuclear medicine studies|
|Most studies using technetium (99mTc)7||< 0.500||> 10|
|Hepatobiliary technetium HIDA scan6||0.150||33|
|Ventilation-perfusion scan (total)||0.215||23|
|• Perfusion portion: technetium6||0.175||28|
|• Ventilation portion: xenon (133Xe)6||0.040||125|
|Iodine (131I), at fetal thyroid tissue6||590.000||‡|
|Environmental sources (for comparison)|
|Environmental background radiation (cumulative dose over nine months)8||0.100||N/A|
Much of our information regarding the effects of radiation in humans has come from the study of atomic bomb survivors who were irradiated with high doses while in utero in Nagasaki and Hiroshima, Japan.2,9,10 Understanding outcomes after high-dose exposure can help physicians understand potential effects from low-dose medical x-rays. These effects can be grouped into three classic categories: teratogenesis (fetal malformation), carcinogenesis (induced malignancy) and mutagenesis (alteration of germ-line genes).
The fetal malformations most commonly caused by high-dose radiation are central nervous system (CNS) changes, especially microcephaly and mental retardation.2 Many Japanese bomb victims who were exposed in utero to doses greater than 10 to 150 rad developed microcephaly.9 A linear, dose-related association between severe mental retardation and radiation was also found, with the important caveat that most cases followed exposure during weeks 10 to 17 of gestation.3,10,11 This trend reaches 40 percent at 100 rad, although it is not statistically significant at doses generated by diagnostic radiographs.3 Nevertheless, until more data are available delineating potential fetal risk, it is prudent to delay non-urgent radiographs during the sensitive period of 10 to 17 weeks of gestation (eight to 15 weeks after conception).
Exposure to as little as 1 or 2 rad has also been associated with a slight increase in childhood malignancies, especially leukemia.2,12 For example, the background rate of leukemia in children is about 3.6 per 10,000.13 Exposure to one or two rad increases this rate to 5 per 10,000.2 While these doses do fall within the range of that supplied by some radiographic studies, the absolute increase of risk (about one in 10,000) is very small. Nevertheless, physicians should carefully weigh the risks and benefits of any radiographic study and include the mother in the decision-making process whenever possible.
RADIATION-INDUCED GENE MUTATION
Radiation can cause germ-line mutations, potentially affecting future generations. Although radiation is commonly believed to create bizarre new mutations, data show that usually it merely increases the frequency of mutations occurring naturally in the general population.3 The dosage required to double this baseline mutation rate is between 50 and 100 rad, far in excess of the radiation doses occurring in common radiographic studies.3,14 Put another way, it is believed that if 10,000 persons were exposed to 1 rad, 10 to 40 new genetic mutations would be induced.14
Table 2 presents various conclusions from key organizations that may help physicians better understand the overall risks from x-rays and other diagnostic imaging methods. Perhaps the most important fact for a physician to remember is that the currently accepted maximum limit of ionizing radiation exposure to the fetus during pregnancy is a cumulative dose of 5 rad.2,5,6,7
|“No single diagnostic procedure results in a radiation dose that threatens the well-being of the developing embryo and fetus.“— American College of Radiology3|
|“[Fetal] risk is considered to be negligible at 5 rad or less when compared to the other risks of pregnancy, and the risk of malformations is significantly increased above control levels only at doses above 15 rad.”— National Council on Radiation Protection5|
|“Women should be counseled that x-ray exposure from a single diagnostic procedure does not result in harmful fetal effects. Specifically, exposure to less than 5 rad has not been associated with an increase in fetal anomalies or pregnancy loss.”— American College of Obstetricians and Gynecologists7|
|Magnetic resonance imaging|
|“Although there have been no documented adverse fetal effects reported, the National Radiological Protection Board arbitrarily advises against its use in the first trimester.”— American College of Obstetricians and Gynecologists and National Radiological Protection Board7|
|“There have been no reports of documented adverse fetal effects for diagnostic ultrasound procedures, including duplex Doppler imaging.” “There are no contraindications to ultrasound procedures during pregnancy, and this modality has largely replaced x-ray as the primary method of fetal imaging during pregnancy.”— American College of Obstetricians and Gynecologists7|
When an expectant mother considers any radiation exposure, the most prominent question in her mind is likely to be, “Is this safe for my baby?” To answer this question, the clinician must carefully choose words that will help a patient understand the real, although very small, risks of exposure. Careful attention must also be given to the parents' potential emotional turmoil at the thought of placing their infant at any increased risk, however small.
For example, the general population's total risk of spontaneous abortion, major malformations, mental retardation and childhood malignancy is approximately 286 per 1,000 deliveries. Exposing a fetus to 0.50 rad adds only about 0.17 cases per 1,000 deliveries to this baseline rate, or about one additional case in 6,000.2,13 However, if numbers like these are quoted to patients, they are likely to hear only the words “risk,” “abortion,” “mental retardation” and “malignancy.” This situation emphasizes the challenge that doctors face in ensuring good communication during counseling.
“Safe” is a relative term, but one that physicians should not be afraid to use. When a radiographic study is needed for appropriate management of a pregnant patient, the American College of Radiology recommends that “health care workers should tell patients that x-rays are safe and provide patients with a clear explanation of the benefits of x-ray examinations.”8 One tool that physicians may consider using to reassure patients is Figure 1, which graphically compares the dosage of radiation provided by various common diagnostic studies or environmental sources with the accepted limit of 5 rad. A patient's particular study could be also plotted on this graph, showing the clear margin of safety that exists for all single diagnostic studies.
Understanding Baseline Risks
As part of counseling, physicians should help patients understand that birth anomalies frequently occur spontaneously, with no identifiable cause. Statistics show that among the general population, in 4 to 6 percent of all deliveries, some spontaneous malformation is present.2 For this reason, it is important never to promise parents a perfect baby. Radiation from diagnostic x-rays is exceedingly unlikely to cause harm to a fetus. Yet, if after any exposure an anomaly is found, a parent's natural inclination may be to blame radiation, and it will then be difficult to help them understand baseline malformation rates. For example, one author reported on a case of a woman who nearly instituted legal action because of mild syndactyly of her infant's fourth and fifth fingers after third-trimester dental radiographs (exposure 0.0001 rad or less).2 This case of syndactyly was almost certainly coincidental, and yet it appeared that the mother had difficulty understanding or accepting this explanation.
Using Reasonable Caution
Diagnostic x-rays during pregnancy are considered safe, yet physicians should use reasonable caution while remaining sensitive to patients' fears and concerns. As with all patient care, good communication promotes a trusting relationship. Unexpected outcomes often lead to anger and legal action. Thus, a factual discussion of the nature of the planned examination and its potential outcomes, and documenting consent are appropriate steps before ordering a study. Asking nonpregnant women with child-bearing potential about the possibility of pregnancy is also an important way to avoid unpleasant surprises.
Women exposed to radiation exceeding a cumulative dose of 5 rad and those with particular concerns about their infant's health may require further evaluation or referral. A radiation physicist can calculate the estimated dose of radiation to the fetus to assist in patient counseling.
A physician's caution should not become unreasonable. Concerns of medicolegal liability may lead some caregivers to inappropriately withhold needed x-rays, thus jeopardizing the health of both mother and fetus. Yet legal liability with exposures less than 5 rad should be minimal and, in fact, many key organizations have declared such exposures to be safe (Table 2). Furthermore, it would be difficult to prove that a given radiograph caused harm in light of the high baseline rate of malformations. Ensuring that radiographs are truly indicated and are ordered in accordance with applicable published guidelines will give further support to a physician's course of action at any review.
If a mother's illness necessitates x-rays, there should usually be no hesitation in ordering the needed study.15 A reasonable guideline has been proposed by the American Academy of Pediatrics and the American College of Obstetricians and Gynecologists (ACOG) that balances these issues. The two groups state that “Diagnostic radiologic procedures should not be performed during pregnancy unless the information to be obtained from them is necessary for the care of the patient and cannot be obtained by other means (especially ultrasound).”16
A woman may fear radiation so much that she believes she should abort a fetus after exposure. Up to 25 percent of exposed women believe their infants are at risk for major malformation.17 After the nuclear reactor accident in Chernobyl, Russia, 23 percent of pregnancies in Greece were terminated because of unfounded concerns about fetal teratogenicity.18 Timely counseling can often correct such a misunderstanding.17 While electively terminating an early pregnancy is legal in the United States, it is important that patients and physicians not confuse social issues with medical ones. Medically, the additional risk imposed by diagnostic radiation is simply too small to justify terminating a pregnancy. For example, one risk associated with lower-dose radiation is childhood leukemia. Yet it would be necessary to abort 1,999 exposed fetuses to prevent one case of leukemia.2 Guidelines from ACOG clearly support this understanding: “Exposure to x-ray during pregnancy is not an indication for therapeutic abortion.”7
A pregnant woman who is ill and requires radiographic imaging faces potential risks from her disease to her own health as well as that of her developing infant's. These risks almost always outweigh the minor hazards posed by low-dose radiation exposure. Physicians should not hesitate to order a study if an appropriate work-up of the mother requires a specific test to guide diagnosis and treatment. However, nonurgent x-rays should be avoided in weeks 10 to 17, the period of greatest CNS sensitivity. When diagnostic imaging is acutely needed, ultrasonography may represent an alternative to ionizing radiation and is considered safe throughout pregnancy. Patient counseling before radiation exposure will help alleviate anxiety and misunderstandings. Proper communication may also reduce unnecessary litigation in the event of an unexpected outcome.