Teratogens are environmental agents or factors, such as substances, chemicals, infections, or maternal conditions, which cause malformations of the embryo, congenital abnormalities, or birth defects. For an agent to be a teratogen, the agent must demonstrate a pattern of effects on the embryo across cases. The various types of effects on the embryo include structural and functional anomalies, prenatal and postnatal growth restrictions, and stillbirth or death of the embryo. The effect on the embryo depends on the type of teratogen, amount of exposure to the teratogen, duration of exposure, and time of exposure in relation to trimester of pregnancy. The embryo tends to be most vulnerable to the harmful effects of teratogens during the first half of the woman’s pregnancy because the first half ...

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