(Death soon after delivery is sometimes due to defect that was in the egg before fertilization. See Unknowns.)
One in 28 deliveries in the U.S. involves some form of birth defect. There are a couple thousand kinds of these defects, most of them not understood, but fortunately today many of them are either correctable like cleft lip, clubfoot and some heart defects or are controllable like PKU or hemophilia. Still, birth defects are the leading cause of infant mortality in the first year, and they too often cause physical or mental disability or lead to early or degenerative death. The following is a partial list of some of the more severe forms of birth defect.
Spinal bifida refers to malformation of the mid region of the spine. It is due to failure of the neural tube to close on day 24 of embryonic development, and occurs in 1 out of 2000 births. This results in control difficulties, brain fluid pressure, learning disabilities depression, latex allergy and early death. The defect is called anencephaly when it is the cephalic or head end of neural tube that fails to form. This occurs more often, in 1 out of 1000 births with incidence 2 to 4 times more common in females than males. The result is more severe - absence of most of the brain, skull, and scalp with the infant either stillborn or dying within a few hours or days after birth. Infants with anencephaly lack forebrain or cerebrum with the remaining brain tissue usually exposed and not covered by bone or skin and often some damage to the brain stem so that the vital functions of breathing and heart regulation may not occur. A baby born with anencephaly is usually blind with eyes but no optic nerve, and has no brain with which to achieve consciousness or feel pain, but has enough spinal column for reflex responses. It is not well understood why the neural tube so often fails to close during early embryonic development, but nutrition is known to be relevant and environment may be a factor. The use of folic acid, a B vitamin, before and during early pregnancy is highly effective, though not a guarantee, for ensuring proper closing of the neural tube. Monte has both a reasonable model and data analysis sowing correlation between neural tube defect incidence and the amount of prenatal consumption of methanol mostly in the form of aspartame.a,b
Chimerism results from the the very rare fusion of two early embryos derived from separate fertilizations, usually of egg and over-sized polar body, but sometimes of the two cells from pre-fertilization cell division of the egg independent of the polar body. Either way, the resulting early embryos are both within the zona pellucida and so likely to fuse. If they manage to develop separately without fusing, the result is half-identical twins. If they do fuse the result is called a human chimera. The reason for the name is that the result can be so fateful. Blood types are complex and about half the time the baby will be a hermaphrodite having both female and male genitals, which is sometimes referred to as intersex.
Down syndrome is caused by an extra third copy of chromosome number 21. In rare cases it can be inherited, but it is usually due to chromosome apportionment error during meiosis I of egg formation. Nature culls some 3/4 of Downs embryos and fetuses, but 1/4 survive to birth, and of those that survive the degree of disability is variable. Down syndrome is characterized by mental retardation, abnormal physical development including short stature, stubby fingers, broad round skull with slanting eyes and large tongue, kidney defects, suppressed immune and sexual systems and malformation of the heart. Females are often fertile, males usually not. With modern medical interventions, median lifetime has increased from 25 years in 1983 to 49 years in 1997. As of 2001, the defect can be detected in the first trimester.
Aneuploidy is any abnormal number of chromosomes, an error that usually originates during meiosis I of egg formation. The term Trisomy is often used to refer specifically to three copies of chromosome 13,(ref) but more generally the term means three copies of any chromosome. Monosomy can also occur. The cases of aneuploidy that sometimes or often survive to birth are:
As of 2008, DNA microarray gene chip technology can detect 150 prenatal genetic disorders.
|Chromosome || % * ||Name ||Survival times**
|Trisomies: || ||
|13 || 2.8 || Patau syndrome ||7 days
|18 || 5.4 || Edwards syndrome ||14.5 days
|21 || 24 || Down syndrome ||25-50 years
|XXY || 53 || Klinefelter syndrome
|XXX || 94 || Tripple X syndrome
|XYY || 100 || XYY syndrome
|Monosomy: || ||
|Y || || Turner syndrome
|Deletions: || ||
|p arm of 1 || ||
|p arm of 5 || ||
*Percent of fertilized eggs with the deficiency that survive to live birth.1,2,3
**Median survival times after live birth(ref)
Sex-Determination Errors: Maleness is determined by the presence of a Y chromosome, which usually means a XY pair. But in one out of 1000 cases there can also be an extra sex chromosome causing XXY, XYY or other combinations. XXY males may have normal lives and have normal sexual function but are usually infertile and may have other symptoms. A XYY combination produces aggressive males 20 times more likely to end up in jail than the rest of the population.
Turner's Syndrome in females results when the error in meiosis produces only a single X sex chromosome. Symptoms usually include short stature, loss of ovulation and major heart defects. The rate of occurrence is one in 2500.
Testosterone Receptor Deficiency (AIS) can affect males with the normal XY pair of sex chromosomes.(Sapolsky, wikipedia) Genetically they are normal males and they do have systemic circulating testosterone. But without receptors for the hormone these males develop physically like girls. At puberty one of two things can happen. If they are totally lacking in testosterone receptors they continue to develop as girls but girls who do not menstruate. If they are only deficient in testosterone receptors, then the increase in testosterone at puberty may cause them to change physically from girl to boy characteristics. This is rare and apparently happens only in cultures that have had a high rate of inbreeding.
Pollution can cause sex change as well. It is well known in amphibians that some pollutants can act like female hormones. This apparently also happens in humans in severe cases. This web site author has lost the reference, but there was a report during the summer of 2006 that a US Indian tribe living across a river from a high-pollutant source was experiencing something like a 60% birth rate of females. Sex change at puberty was not reported. If someone has the reference, please forward it.
Homosexuality is not a form of chimerism, and it may not be related to any form of birth defect. An October, 2004, study report headed by Andrea Camperio-Ciani from the University of Padua has found that male homosexuality is linked to the X chromosome. But a previous different study suggesting genetic link could not be confirmed by other researchers.
Huntington's disease is an inherited degeneration of motor control structures of the brain that starts at age 40 to 50 and leads to death in about five years after onset. It affects about 1 in 20,000 in the U.S., but chance of occurrence is 50 % if a parent was affected. There is no diagnostic test, at least not yet even though the relevant defective gene and protein have recently been isolated. The hallmark symptom is uncontrolled flailing of limbs, but the disease starts with subtle irritability, figitiness and sudden falls, and progresses to loss of all body control including speech and facial expression and then dementia and death. There is an error of DNA replication during meiosis, too many codes for a particular amino acid being repeated too often. Practically all incidents of this disease in the eastern U.S. are descendants of two Suffolk, England, ancestors who emigrated to Salem, Massachusetts in 1630. The main cause of deranged "witch" behavior in Salem is now accepted by many to have been due to a mold in rye grain during damp years that produces LSD-like compounds. But it is likely that some of the women executed as witches in Salem may have been displaying the symptoms of Huntington's disease.
Tay-Sachs disease is an inherited inability to produce a fat metabolism enzyme necessary in nerve cells. This causes central nervous system degeneration, progressive mental deterioration, blindness, paralysis and/or epileptic seizures, with death usually between ages 3 and 5.
Cystic fibrosis is an inherited disruption of chloride transfer across cell membranes causing water retention, cysts and eventual blockage of pancreas, lungs and liver. Age of death is 25-30.
Muscular dystrophy is a mother inherited inability of sons to produce essential skeletal muscle protein called dystrophin. Symptoms start with leg weakness before age 3 and progressively worsen until death by age 30.
Hemophilia is another mother inherited trait. Blood fails to clot.
Sickle cell anemia is still another inherited trait. There is an error in one amino acid code in the hemoglobin gene, and as a result the hemoglobin produced has the wrong shape causing high viscosity for blood flow. It turns out that the hemoglobin gene has two parts, one used during fetal development and the other used later during adult life. A modern DNA cure for sickle cell anemia seeks to take advantage of this by reactivating the fetal gene code.
PKU is an inherited inability to process a certain protein which then accumulates in the blood and causes brain damage. If diagnosed early enough the brain damage can be stayed by stringent control of diet.
Cerebral palsy: Not diagnosed until age 2 to 3, but 70 % due to problems in place before birth. Rate of incidence is 1 in 300.
Fetal Alcohol Syndrome: 1 in 1000 - physical and mental defect likely if drinking heavy during pregnancy but still a significant risk with light drinking during pregnancy.
Sexually transmitted diseases: 1 in 2000 - can result in bone defects, stillbirth or newborn death.
There are many informative web pages on birth defects, such as the one by the March of Dimes.