Environmental factors

The effect of teratogens depends on several factors: the overall sensitivity of the embryo, which is given by its genotype; the sensitive period (critical period) through which developing structures undergo; the dosage of the teratogen. The sensitivity of a structure is generally highest in the period during which it undergoes rapid growth and becomes organized. This explains why most anomalies arise in the embryonic period. However, the embryo is also at risk during the fetal period, and certain systems have their sensitive period after birth.

Teratogenic environmental factors can be infections, maternal diabetes, medications, hormones, drugs, chemicals, and physical factors.
Infections. Many infectious diseases can lead to developmental abnormalities. This is particularly the case for rubella (German measles) if it appears in the early stage of pregnancy. The rubella virus can lead to cataract, deafness and cardiac defects (persistent ductus arteriosus). Women who have never had rubella can nowadays be immunized by vaccination. Cytomegaly virus can lead to microcephaly, deafness, hepatosplenomegaly and behavioural impairment. Herpes simplex may lead to microcephaly, hepatosplenomegaly and mental retardation. With chickenpox (varicella), malformations of the limbs and muscular atrophy can be observed. Toxoplasmosis leads to chorioretinitis and to different forms of visual-impairment. Syphilis may result in deafness, mental disability and diffuse fibrosis of different organs.
Maternal diabetes warrants particular mention. It can lead to a large variety of developmental anomalies involving both the cardiovascular and the skeletal system.