Abdominal Wall Defects
These disorders are also known as ventral wall defects. There are two serious but rare types of abdominal wall defects.

• Omphalocele is a defect in which the intestines of the fetus develop outside the body. If the defect in the abdominal muscle is large, other internal organs such as the liver and bladder may also develop outside the fetal abdomen. About half of all infants with omphalocele have other malformations either directly related to the digestive tract or affecting another organ, such as the heart.
• Gastroschisis is a defect in which the intestines and occasionally other organs protrude through the defect, usually on the right side of the fetal abdomen. Gastroschisis is rarely associated with other malformations and can usually be corrected by surgery after birth.

Chromosomal Abnormalities
These disorders occur when the fetus possesses more than the normal 46 chromosomes. Chromosomes carry the genetic information that controls the way the body grows and develops.

• Down syndrome is caused by an additional chromosome 21. This extra chromosome always causes mental retardation. Frequently, it also causes heart defects and other abnormalities. Almost all individuals with Down syndrome require care throughout their lives, though some are able to work in sheltered workshops.
• Trisomy 18 is caused by an additional chromosome 18. 95 percent of these trisomic fetuses abort spontaneously. Live-born infants are severely growth retarded and seldom survive more than two months after birth. A few have lived into their teens. Severe cardiac problems are common. Other problems include microcephaly, small jaw, long skull, horseshoe kidney, single umbilical artery, club hands, and rocker-bottom feet.

Neural Tube Defects
These are defects of the central nervous system, which include the spinal cord and the brain. In the developing embryo, this system is called the neural tube. Normally the neural tube closes completely. If, however, all or part of the neural tube fails to close, the baby has a neural tube defect. In some instances, the opening in the neural tube is exposed. This is known as an open neural tube defect and is often detected by the AFP screen. In other instances the neural tube defect is covered with bone and skin. This is known as a closed neural tube defect and in most cases would not be detected by the AFP test.

Open Neural Tube Defects

• Anencephaly
  This disorder occurs when most of the head, brain, and possibly the spinal cord do not develop normally. Newborn children with this severe disorder usually die shortly after birth, as this condition is incompatible with life.
• Encephalocele
  This disorder results in a hole in the skull through which brain tissue protrudes. Although most babies with encephalocele do not live or are severely retarded, a few children have survived because of corrective surgery completed shortly after birth.
• Spina bifida
  This disorder, also known as cleft or open spine, is a defect of the spinal column. The nerves of the spinal cord do not connect properly with the nerves in the lower part of the body, causing problems with moving lower body muscles. Normal development of the spina bifida child with little physical handicap is possible. Often, however, paralysis of the lower limbs, repeated urinary tract infections, hydrocephalus, and incontinence occur.
  The spinal malformation and nerve damage caused by this disorder hamper movement in varying degrees. Some individuals can walk by themselves. Others use braces and crutches or wheelchairs. Modern surgical techniques can help many children born with spina bifida lead healthy and productive lives. There are three types of spina bifida:
  1. Spina bifida occulta is an opening in one or more of the bones of the spinal column that does not involve any damage to the spinal cord.
  2. Meningocele is a more serious form of spina bifida. The protective covering around the spinal cord (meninges) pushes out through the opening in the spinal column. The sac containing the exposed part of the spinal cord is called the meningocele. Early surgery can usually repair this problem with little or no damage to the nerves in the spinal cord.
  3. Myelomeningocele is the most severe form of spina bifida. The spinal cord containing the deeper nerves of the spinal column protrudes from the open spine without a protective covering of the skin. Infection can be a serious problem because of leaking spinal fluid. This defect usually occurs at the lower end of the spine resulting in paralysis of the legs along with poor bladder and bowel control.