Timing of teratogen risk in pregnancy
A teratogen (anything that can disturb normal development) does the most structural damage, abnormal formation of an organ, in the embryonic stage, roughly weeks 3 to 8 after fertilisation.
Remember
Structural damage to organs comes mainly from exposure in the embryonic stage, when the organs are forming, yet the central nervous system (the brain and spinal cord) stays at risk throughout pregnancy.
Memory aid
a building site: the frame goes up first, so mistakes made then are structural, whereas later work is fitting out and finishing
Picture organ formation as the frame of a building going up first, with the later months as fitting out and finishing.
A teratogen is anything that can disturb normal development, such as certain medicines, alcohol or an infection. The damage it does depends on the timing of the exposure, because the organs that are forming or developing change as pregnancy goes on.
The period of highest risk for structural damage is the embryonic stage, roughly weeks 3 to 8 after fertilisation. This is when organogenesis (the formation of organs) takes place, as rapidly dividing cells specialise and lay down the major organ systems. A harmful exposure then can leave a permanent defect in whichever organ is forming at that moment, which is called a structural malformation (abnormal formation of an organ). Prescribers take most care in early pregnancy because the organs are forming then, and exposure may happen before a woman knows she is pregnant.
Before implantation the pattern is usually all or nothing: the embryo is either lost or develops normally. From week 9 the foetal period begins. Most organ systems already exist by then and mainly need to grow and mature, so they are at lower risk of structural damage.
The exception is the central nervous system (the brain and spinal cord). It develops over a very long time, so it stays vulnerable throughout pregnancy. By 20 weeks most organs have already formed, so an exposure is less likely to cause a major malformation of another organ, but it can still impair brain development, which may only show later as problems with learning, thinking or concentration. Foetal alcohol spectrum disorder (a lifelong condition caused by alcohol exposure in the womb) is an example.
Many teratogens cross the placenta, but crossing does not decide which organs are harmed: timing does.
Key facts
- Teratogen
- anything that can disturb normal development, such as a drug, alcohol or an infection.
- Highest-risk window for structural damage
- the embryonic stage, about weeks 3 to 8 after fertilisation, when the organs form.
- Before implantation
- the pattern is usually all or nothing, with either loss of the embryo or normal development.
- Foetal period
- from week 9 most organs are growing and maturing, so the risk of structural damage is lower.
- Exception
- the central nervous system develops for much longer, so it stays vulnerable throughout pregnancy.
- Placenta
- many teratogens cross it, but timing, not crossing, decides which organs are harmed.
- In pregnancy
- a prescribed medicine should not be stopped without checking with a doctor.
Exam traps
- Growth in the foetal period does not make it the time of greatest structural risk, because most organs already exist, so teratogens usually cause problems with growth or function rather than major structural malformations.
- Many teratogens cross the placenta, but crossing does not make every stage of pregnancy equally dangerous, because the risk depends on which organs are forming or developing at the time of exposure.
- The central nervous system does not become as resistant as other organ systems once the embryonic stage is over, because it develops over a much longer time and stays at risk throughout pregnancy.
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