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Radiation Effects on the Embryo and Fetus
Timing sets the effect: all-or-none in weeks 0–2, malformations in weeks 2–8, CNS and growth effects later. Routine radiographs stay far below thresholds.
Why the embryo is sensitive
The law of Bergonié and Tribondeau says cells are most radiosensitive when they divide rapidly and are undifferentiated, or not yet specialized. The embryo is made almost entirely of such cells. Early on, each cell will give rise to whole groups of tissues, so damage to a few cells can matter a great deal. Sensitivity is highest early in pregnancy and falls as organs form and mature. The weeks in this lesson are counted from conception. Clinicians usually date a pregnancy from the last menstrual period, which adds about 2 weeks, so a stage that starts 8 weeks after conception is about 10 weeks by dates.