Your Pregnancy After 35 : How Age Affects Your Pregnancy (part 3) - Chromosomal Abnormalities

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8. Understanding Basic Genetics

Human cells contain 46 chromosomes each (diploid number); each sperm and each egg contain 23 chromosomes (haploid number). Chromosomes are composed of DNA and proteins that contain genes. Each cell contains 23 pairs of chromosomes—one chromosome of each pair is inherited from each parent.

Of the 23 pairs, 22 are autosomes (nonsex chromosomes) and one pair is the sex chromosomes, X and Y. In a normal woman, the sex chromosomes are represented by two X (XX) chromosomes, and in the normal man by X and Y (XY) chromosomes.

Chromosome analysis usually involves blood cells. Chromosomes are viewed through a microscope, then photographed and enlarged. They are then arranged in pairs, called a karyotype; a karyotype is used to identify abnormalities in the number of chromosomes.

Healthcare providers can examine individual chromosomes for abnormalities. We have learned that too much or too little chromosome material can cause an abnormality.

9. Incidence of Genetic Disease

The hoped-for and anticipated outcome for every pregnancy is a normal, healthy baby. It is expected and taken for granted. However, that is not always the case. We believe nearly half of all first-trimester miscarriages are caused by some type of chromosomal abnormality. About 1% of all babies born have a chromosomal abnormality; 4 to 7% of perinatal deaths (death after birth) are attributed to chromosomal problems.

Among newborns, 3 to 5% are born with a major birth defect, such as a heart defect; many of these birth defects have a genetic component. In the United States, this represents as many as 225,000 children each year.

Mental retardation is another area of concern. It is estimated that genetic problems cause 70% of mental retardation. Genetic counseling may offer an indication that this might occur.

10. Chromosomal Abnormalities

Chromosomal abnormalities can occur in any cell, and they can occur as an abnormal chromosome number or as an abnormality in the structure of the chromosome itself. With Down syndrome, an individual has 47 chromosomes instead of the normal 46. With Turner’s syndrome, the individual has 45 chromosomes.

A loss, a gain or a repositioning of chromosome material identifies a structural abnormality. Terms used to describe these various conditions include additions, duplications, deletions, inversions and translocations.

A couple of possible causes of chromosomal abnormalities are advanced parental age and radiation exposure of mother, father or fetus, which can affect gene regulation. Advanced parental age usually refers to the mother’s age, but some research has also implicated the father’s age. Age is a factor for the mother because we believe females are born with all the eggs they will ever have. Women are exposed to radiation and other teratogens in the environment throughout their lives. These exposures have an additive effect—the more teratogens a woman is exposed to, the greater the chance of damaging her eggs.

Radiation exposure, such as X-rays, can damage genetic material and affect gene function and regulation. Genes control development of the fetus, its organs and growth. Three of the most common examples of chromosomal abnormalities are discussed below.

Down syndrome is the most common chromosomal abnormality; it occurs in about 1 in 800 births. An extra chromosome causes this syndrome. Those born with Down syndrome can live fairly long lives and may be moderately to severely mentally retarded. About half have congenital heart disease; particular physical abnormalities are common to the syndrome.

Trisomy 18 (Edwards’ syndrome) is a severe chromosome abnormality occurring in approximately 1 in 6000 births. An abnormal chromosome 18 causes the syndrome. Babies with trisomy 18 have multiple abnormalities of major organs.

Trisomy 13 (Patau’s syndrome) occurs about once in 5000 births. An extra chromosome 13 causes the syndrome. Babies born with trisomy 13 may have multiple abnormalities.

Sex-Chromosome Abnormalities

Abnormalities of sex chromosomes are relatively common—1 in every 500 births. It is believed this type of abnormality causes about 25% of all miscarriages. We discuss three of these syndromes below.

Turner’s syndrome occurs about once in every 10,000 female births. It is the most common chromosome abnormality identified in miscarriages. Instead of having two X chromosomes, a baby girl born with Turner’s syndrome has only one X chromosome, called 45X. A mature girl with Turner’s syndrome is short, sexually underdeveloped, with a webbed neck, very small ovaries and often heart and kidney problems. The condition may not be identified until she reaches puberty.

Klinefelter’s syndrome is found in approximately 1 in every 1000 males. The most common characteristic is very tall stature when the boy reaches maturity. These boys are born with an extra sex chromosome, thus the 47XXY label. A boy is underdeveloped sexually and has small testes.

Fragile-X syndrome (also called X-linked) is one of the most common causes of mental retardation in males. Major physical characteristics include large ears, large hands and language-development delays.

Congenital Infections

Rubella was one of the first maternal infections researchers identified as the cause of fetal malformations. Other infections that can cause malformations include cytomegalovirus (CMV), toxoplasmosis, herpes simplex, parvovirus 19 and syphilis.

Congenital infections can cause problems in a baby. These range from major birth defects, such as heart defects, to newborn infection at birth.

Velocardiofacial Syndrome (VCFS)

Velocardiofacial syndrome (VCFS) is a genetic condition that may be hereditary. It is known by many names, including Shprintzen syndrome, craniofacial syndrome and conotruncal anomaly face syndrome. VCFS is one of the most common syndromes in humans, second only to Down syndrome in frequency.

Health Risks of Older Pregnant Women by Age

Risk 30-34 35-39 40+
Uterine bleeding  8%  9%  9%
Diabetes 35% 48% 65%
Heart disease  7%  8%  9%
Hypertension 34% 38% 47%

VCFS is characterized by various medical problems. The immune system, endocrine system and neurological system may be involved. Symptoms do not all occur 100% of the time. Most people with VCFS exhibit a small number of problems; many problems are relatively minor.

The exact cause of velocardiofacial syndrome is unknown; however, investigators have identified a chromosomal defect in people with VCFS. Most children who have been diagnosed with this syndrome are missing a small part of chromosome 22.

It is estimated that VCFS is inherited in only 10 to 15% of cases. Most of the time neither parent has the syndrome nor carries the defective gene. The occurrence of congenital heart disease is most often the leading factor in diagnosis.

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