Radiosensitivity, as measured by the risk of inducing a congenital malformation, begins to decline after 45 days post-conception – risk of congenital abnormalities becomes very small for most organ systems by the second and third trimesters. 

The CNS, however, is at risk from 8-10 weeks GA through birth (no  evidence for impairment if exposed before 8-10 weeks). 

Because of such a long period of vulnerability, it is little surprise that structural and functional anomalies of the CNS are the most frequent sequealae of embryonic or fetal irradiation.  Microcephaly may occur with or without mental impairment.  

The cerebellum is particularly radiosensitive.  Regarding cognitive effects, attempts to quantify vulnerability suggest a 30 IQ point loss per Sv.

Fetal Thyroid

Iodine-131 readily crosses the placenta. By the 11th week of gestation the fetal thyroid begins to take up iodine (tracer doses given before 8-10 weeks fetal age have had no detectable effect on the fetal thyroid).  Advise thyroid function study in neonate if iodinated contrast is used during pregnancy.

Late pregnancy exposure – potential effects

Animal studies indicate that general growth retardation and marked hypoplasia of the gonads may be seen later in life. Smaller degrees of development arrest of lung, liver, heart, and kidney and changes in the thymus and spleen have also been observed.