Medications, hormones, drugs, chemicals. There is a large variety of harmful substances in this category but only a few are described here. However, the importance of the ones not mentioned should not be neglected.
Excessive alcohol consumption by the mother leads to fetal alcohol syndrome, which is characterized by craniofacial anomalies, deformations of the limbs, cardiovascular abnormalities and mental disability. It is thought that even small amounts of alcohol may be harmful, though their effect is at first not noticed.
Antihypertensiva of the Angiotensin-Converting Enzyme Inhibitor type (ACE-Inhibitor), if given during the middle and last third of pregnancy, can lead to growth retardation, impairment of renal function and to oligohydramnion (less amniotic fluid than normal).
Thalidomide at first seemed to be a harmless medication. It was taken by pregnant women in the 1950s to help with morning sickness and as a sleeping aid. However, after some years it turned out that it quite often led to malformations of the limbs (amelia and meromelia). The period from day 21 to 36 following ovulation is the critical period for impairments in limb development. This medication seems to have different effects in different species. Unlike its effect in primates, it does not seem to cause any malformations in mice and rats.
Physical factors. High doses of ionizing radiation lead to malformations. Small doses for diagnostic radiography, such as X-rays, seem to be harmless. Mechanical factors can be intra-uterine compression and amniotic bands, which arise relatively frequently after disruption of the amnion. These bands can cause different malformations such as amputation of limbs, fusion of fingers or constriction of the umbilical cord.