<P> For multicellular organisms that develop in a womb, the physical interference or presence of other similarly developing organisms such as twins can result in the two cellular masses being integrated into a larger whole, with the combined cells attempting to continue to develop in a manner that satisfies the intended growth patterns of both cell masses . The two cellular masses can compete with each other, and may either duplicate or merge various structures . This results in conditions such as conjoined twins, and the resulting merged organism may die at birth when it must leave the life - sustaining environment of the womb and must attempt to sustain its biological processes independently . </P> <P> Genetic causes of congenital anomalies include inheritance of abnormal genes from the mother or the father, as well as new mutations in one of the germ cells that gave rise to the fetus . Male germ cells mutate at a much faster rate than female germ cells, and as the father ages, the DNA of the germ cells mutates quickly . If an egg is fertilized with sperm that has damaged DNA, there is a possibility that the fetus could develop abnormally . </P> <P> Genetic disorders or diseases are all congenital, though they may not be expressed or recognized until later in life . Genetic diseases may be divided into single - gene defects, multiple - gene disorders, or chromosomal defects . Single - gene defects may arise from abnormalities of both copies of an autosomal gene (a recessive disorder) or of only one of the two copies (a dominant disorder). Some conditions result from deletions or abnormalities of a few genes located contiguously on a chromosome . Chromosomal disorders involve the loss or duplication of larger portions of a chromosome (or an entire chromosome) containing hundreds of genes . Large chromosomal abnormalities always produce effects on many different body parts and organ systems . </P> <P> A low socioeconomic status in a deprived neighborhood may include exposure to "environmental stressors and risk factors ." Socioeconomic inequalities are commonly measured by the Cartairs - Morris score, Index of Multiple Deprivation, Townsend deprivation index, and the Jarman score . The Jarman score, for example, considers "unemployment, overcrowding, single parents, under - fives, elderly living alone, ethnicity, low social class and residential mobility ." In Vos' meta - analysis these indices are used to view the effect of low SES neighborhoods on maternal health . In the meta - analysis, data from individual studies were collected from 1985 up until 2008 . Vos concludes that a correlation exists between prenatal adversities and deprived neighborhoods . Other studies have shown that low SES is closely associated with the development of the fetus in utero and growth retardation . Studies also suggest that children born in low SES families are "likely to be born prematurely, at low birth weight, or with asphyxia, a birth defect, a disability, fetal alcohol syndrome, or AIDS ." Bradley and Corwyn also suggest that congenital disorders arise from the mother's lack of nutrition, a poor lifestyle, maternal substance abuse and "living in a neighborhood that contains hazards affecting fetal development (toxic waste dumps)." In a meta - analysis that viewed how inequalities influenced maternal health, it was suggested that deprived neighborhoods often promoted behaviors such as smoking, drug and alcohol use . After controlling for socioeconomic factors and ethnicity, several individual studies demonstrated an association with outcomes such as perinatal mortality and preterm birth . </P>

When are hereditary defects present in a developing fetus
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