Every organ forms on its own schedule. Teratology models each organ's vulnerability as a susceptibility function of gestational week S(t), which is highest while its cells are actively dividing, migrating and fusing (organogenesis, roughly weeks 3–8) and falls off once the organ's basic structure is established:
S(t) = 1 if peakStart ≤ t ≤ peakEnd
S(t) = smooth(1 − (peakStart − t)/preTail) if t < peakStart (rising)
S(t) = max(smooth(1 − (t − peakEnd)/postTail), baseline) if t > peakEnd (falling)
smooth(x) = x²(3 − 2x) (cubic smoothstep, x clamped to [0,1])
The heart's looping tube and the neural tube close first (weeks 3–6), so they are the earliest and often the most severely affected — this is why a missed period in early pregnancy is already inside several organs' critical windows before it is even confirmed. The brain keeps a low non-zero baseline susceptibility for the rest of gestation because neuronal migration and synaptogenesis continue long after the neural tube itself has closed, which is why alcohol and some anticonvulsants remain risky in later trimesters even though they no longer cause structural malformations.
- Week slider / Play / drag the marker on the timeline — scrubs the exposure week; every organ's band highlight and risk bar update live from its own S(t) curve.
- Mark exposure — records a teratogen exposure at the current week; every organ with S(t) > 0.5 at that moment is flagged as affected and its contribution S(t) is added to the cumulative risk score.
- Cumulative risk — the sum of S(t) across all marked exposures and all organs, a simple proxy for how much total organogenesis was exposed, not a real clinical risk estimate.
Real-world relevance: this is the same shape of chart (Moore & Persaud's "critical periods of human development") that underlies prenatal counseling — why isotretinoin, valproate, warfarin and rubella infection are dated by gestational week, and why avoiding known teratogens in the first trimester matters most.