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Early pregnancy can go undetected, so it is prudent to ensure that patients of childbearing potential are not pregnant before undergoing [[Application_of_ionising_radiation_in_healthcare#Diagnostic_Radiology|diagnostic radiology]] or [[Application_of_ionising_radiation_in_healthcare#Nuclear_Medicine|nuclear medicine]] studies that provides doses above which the risk of adverse fetal health effects is not considered negligible (1–10 mGy).<ref>PRACTICE PARAMETER FOR IMAGING PREGNANT OR POTENTIALLY PREGNANT ADOLESCENTS AND WOMEN WITH IONIZING RADIATION[https://www.acr.org/-/media/ACR/Files/Practice-Parameters/Pregnant-Pts.pdf]</ref> Before [[Radiation Application_of_ionising_radiation_in_healthcare#Radiation_Therapy|radiation therapy]], and in the absence of a documented history of applicable gynaecological surgery (e.g. tubal ligation, hysterectomy) or an established postmenopausal state, serum or urine pregnancy tests should be obtained, ideally 24–72 h prior to treatment.<ref>The SNMMI Practice Guideline for Therapy of Thyroid Disease with 131I[http://jnm.snmjournals.org/content/53/10/1633]</ref>

There are radiation-related risks to the embryo/fetus during pregnancy that are related to the stage of pregnancy and the absorbed dose to the embryo/fetus. [[ICRP]] has evaluated the effects of prenatal irradiation in detail.<ref name="Pub090">[[ICRP Publication 90]] Biological Effects after Prenatal Irradiation (Embryo and Fetus). Ann. ICRP 33(1-2), 2003.</ref> These effects include lethal effects, malformations, central nervous system effects, leukaemia and childhood cancer. Consideration of these effects is important when pregnant patients undergo [[Application_of_ionising_radiation_in_healthcare#Diagnostic_Radiology|diagnostic radiology]], [[Application_of_ionising_radiation_in_healthcare#Interventional_Procedures|interventional procedures]], or [[Application_of_ionising_radiation_in_healthcare#Radiation_Therapy|radiation therapy]] using ionising radiation. A balance must be attained between the health care of the patient and the potential for detrimental health effects to the embryo/fetus that accompanies the specific radiological procedure.<ref name="Pub084">[[ICRP Publication 84]]Pregnancy and Medical Radiation. Ann, ICRP 30(1), 2000.</ref>

For [[Application_of_ionising_radiation_in_healthcare#Diagnostic_Radiology|diagnostic radiology]], referring physicians should have discussions with imaging specialists to help determine the best tests for their pregnant patients, taking into account non-ionising diagnostic imaging modalities, such as ultrasound or magnetic resonance imaging. If these modalities are not the most appropriate test based on the clinical scenario, or are not available, the most appropriate ionising radiation imaging modality should be utilised in such a way as to achieve the diagnosis required while keeping fetal and maternal doses as low as reasonably possible. Almost always, if a [[Application_of_ionising_radiation_in_healthcare#Diagnostic_Radiology|diagnostic radiology]] examination is medically indicated, the risk to the mother of not performing the procedure is greater than the risk of potential harm to the embryo/fetus.

[[Radiation Application_of_ionising_radiation_in_healthcare#Radiation_Therapy|radiation therapy]] (whether with external beam, brachytherapy, or nuclear medicine) can involve high radiation doses and may cause harm to the fetus. The risk to the fetus is dependent on the stage of fetal development. Lethal risk occurs in the pre-implantation phase, malformation occurs during major fetal organogenesis 3–7 weeks post implantation, mental retardation occurs at 8–15 weeks post implantation, and future cancer risk follows a stochastic model. Although there is variability in threshold radiation doses, it can be generalised that risks become notable at fetal doses of 100 mGy (10 rad) or above. So, it is essential to ascertain whether a female patient is pregnant prior to [[Radiation Application_of_ionising_radiation_in_healthcare#Radiation_Therapy|radiation therapy]]. In pregnant patients, cancers that are remote from the pelvis can usually be treated with radiation therapy. However, this requires careful planning. Cancers in the pelvis cannot be treated adequately during pregnancy without severe or lethal consequences for the embryo/fetus. [[Radiation Application_of_ionising_radiation_in_healthcare#Radiation_Therapy|radiation therapy]] in pregnant patients requires pre-therapy fetal dosimetry estimation followed by a comprehensive discussion of the benefits and risks of the procedure, with the patient included as part of the informed consent process.