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Thall Bastow BD, Holmes JL (23 February 2016). "Teratology and drug use during pregnancy". Medscape. WebMD. Retrieved 24 February 2016. - Teratogens are drugs that may cause birth defects via a toxic effect on an embryo or fetus

The teratogenic effects of medications vary temporally. The fetus' susceptibility to injury depends on its period of development. Different organs have different critical periods, though the span from gestational day 15 to day 60 is critical for many organs. The heart is most sensitive during the third and fourth weeks of gestation, whereas the external genitalia are most sensitive during the eighth and ninth weeks. The brain and skeleton are sensitive from the beginning of the third week to the end of pregnancy and into the neonatal period.

Genetic defects and medications can cause similar abnormalities, such as those resulting from warfarin and Happle syndrome.

Several studies are related to axial defects in mice. In mice, axial malformations can result from mutations in certain HOX genes or from exposure to retinoids.

Happle syndrome, or human X-linked dominant chondrodysplasia punctata (CDPX2; see Skeletal Dysplasia), is associated with mutations in the human emopamil-binding protein, a delta-delta-sterol isomerase involved in cholesterol biosynthesis. Happle syndrome is a genetic disease of bone and cartilage characterized by defective bone mineralization, telebrachydactyly, and facial dysmorphism with nasal hypoplasia. Maternal ingestion of warfarin can result in a fetal phenotype similar to that of this syndrome.

Teratogens, such as cyclophosphamide, result in fetal demise due to excessive apoptosis.

Drug Exposures in the Male Partner

Research is increasingly addressing the role of paternal exposure to medications before conception or during his partner’s pregnancy. Certain exposures may alter the size, shape, performance, and production of sperm. This observation suggests that drug exposure in the male may put the fetus at risk. Animal studies have shown that paternal teratogenic exposure may lead to pregnancy loss or failure of the embryo to develop. However, unlike teratogenic agents affecting pregnant woman, teratogenic agents affecting the father do not seem to directly interfere with normal fetal development. Animal studies showing that paternal teratogenic exposure may lead to pregnancy loss or embryonic failure.

....agents such as recreational drugs do affect sperm quality and, to a limited degree, indirectly expose the developing fetus to the substance. Rather than affecting the developing fetus, teratogens like drug and alcohol seem to lower the likelihood of a woman's becoming pregnant.

Paternal alcohol use may increase the risk of heart defects in newborns. In one study, paternal smoking was associated with heart defects. Chemotherapy or radiation therapy to treat cancer in a father may increase the risk chromosomal abnormalities of the fetus. Studies have demonstrated less-than-normal numbers of chromosomes and damage to the structure of chromosomes in the sperm of men with cancer. ......

FDA Rating System for the Teratogenic Effects of Drugs

The FDA, the government agency that oversees the safety of drugs, provides the most widely used system to grade the teratogenic effects of medications. The FDA assigns a safety category for medications by using a 5-letter system: A, B, C, D, and X. This safety category must be displayed on the labels of all drugs.