Introduction

What is a teratogen?

Teratogens are agents that cause fetal abnormalies during development. The word teratogen comes from the greek for monster. A Teratogen can be infectious or chemical. During developoment these agents are able to cross the placenta and disrupt patterning or cell signalling in various ways.

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What can determine if a fetus will be harmed?

• ·         The length of exposure
• ·         Amount of exposure (ie Dose)
• ·         The age of the fetus

will effect what organs or tissues are damaged and how severe a particular teratogen is. It can also depend on the maternal metabolic rate as this will determine how much of a harmful substance is in the body for how long. Genetic differences in the fetuses may also play a role; some fetuses may be be more susceptible to various agents.

Organogenesis mostly occurs during the first trimester and so exposure to teratogens can be particularly harmful during this stage. The brain continues to develop through out embryogenesis and so metal retardation as a result of teratogen exposure can be less time dependant. Other examples show that exposure between day 35 and 37 of gestation results in no ears, between day 39 and 41 results in no arms or exposure at day 41 to 43 results in no uterus. During any pregnancy there is a 3-5% chance that the baby will have a deformity. There is also no absolute teratogen that will will cause the same defect all of the time.

How do you find a teratogen?

Teratogens are discovered if there is an increased prevalence of an unusual phenotype in a particular area, after specific druf administration or after a particular disease epidemic. Studies are then undertaken to link the cases together and then these links are explored. For example in Minatama Bay, Japan it was discovered that pregnant women ingested fish contaminated with methyl mercury. This chemical caused an encephalopathy that become known at Minatama disease.

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How do you the risk to the fetus?

There are various terms used to describe the risk to the baby.

The absolute risk is the chance of the baby having a defect after exposure to the agent, the relative risk is the risk of an exposed fetus compared to a non exposed fetus and the attributal risk is the amount of babies with the same defect being caused by the same teratogen.

 

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