Organogenesis occurs predominantly between 2 and 15 weeks gestation. This is the period when the fetus is most susceptible to the teratogenic effects of ionizing radiation, which include microcephaly, microphthalmia, mental retardation, growth retardation, behavioral defects, and cataracts. Teratogenic effects are extremely unlikely in fetuses before 2 weeks of gestation and after 15 weeks of gestation [1]. Teratogenesis is considered a non-stochastic effect of radiation (i.e., a threshold dose exists below which there is no risk). The threshold radiation dose below which no teratogenic effects occur is not known, but is estimated to range from 5 to 15 rad [2]. The radiation dose to the fetus from a spiral CT study of the maternal pelvis using typical technical parameters is variable and depends on gestational age and scanning parameters such as slice thickness and mAs. That said, estimated doses range from 2.4 rad in the first trimester to 4.6 rad in the third trimester [3, 4]. An older study that is probably not representative of current technology suggested fetal doses of up to 5-10 rad [5]. Therefore, the radiation dose of pelvic CT is likely at or below the estimated threshold level for induction of congenital malformations. In practice, studies have shown the incidence of malformations is not measurably increased after in utero irradiation in humans [6].
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