DESPITE ITS apparently isolated containment, the developing individual is affected by the rich, diverse environment of the womb. The mother’s body sends hormonal signals to the fetus that fluctuate with diurnal rhythms and maternal effects, such as stress. Stimuli experienced by the mother are transmitted to the fetus with varied degrees of filtration. For example, small molecules (e.g., alcohol, simple sugars) tend to pass easily through maternal circulation to the child, but larger ones (e.g., larger proteins) do not. External sounds are dampened in varied degrees, depending on the frequency of the sound, whereas internal sounds are transmitted well. Light is almost completely blocked. Environmental inputs might be damaging (e.g., alcohol, viruses) or supportive (e.g., nutrients, movement, voice, etc.). The developing embryo and fetus is susceptible to damage by environmental factors in two major ways: direct insult to fragile, immature tissues and perturbations of the developing system that alter the trajectory of development (e.g., damage to cells at the tip of the limb bud can stop development of the limb). From a positive perspective, the sensory and hormonal environment of the uterus supports embryonic and fetal development, especially the development of nervous system function.