Timing of Exposure & Type of
Birth defect ?
The susceptibility of the conceptus to teratogenic
agents varies with the developmental stage at the time exposure. This concept
of critical stages of developmental is particularly applicable to alterations
in structure. It is during the second to the eighth weeks of development after
conception – the embryonic period – that most structural defects occur. For
such defects it is believed that there is a critical stage in the developmental
process after which abnormal embryogenesis cannot be initiated.
NTD occurs when –at what period of gestation??
Ans Neural tube defects result from the
failure of the neural tubes to close. Given that this process occurs between 22
and 28 days post conception any exogenous effect on development must be present
at or before this time.
The neural
tube has five distinct closure sites that may respond differentially to agents
and may respond differently in timing. Investigation of thalidomide
teratogenicity have clearly shown that effects of the drug differ as function
of the developmental stage at which the pregnant woman took it.
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