Women's Healthcare Topics

Genetic Disorders and Birth Defects

Reviewed by James Brann, M.D.

Worried about Birth Defects?

You are not alone. Every parent worries about their baby being born healthy and without birth defects. This worry is a normal concern and commonly expressed by parents at many doctor's visits. Most babies are born healthy, but two out of every one hundred born do have a birth defect. Many babies that are born with a defect have parents that don't have any known risk factors and the cause for the defect isn't known.

In order to understand genetic disorders you should first understand the basics of genetics.

First Important Point: The man contributes 23 chromosomes (sperm) and you contribute 23 chromosomes (egg), which combine at fertilization forming the first cell that will form the baby (a total of 46 chromosomes).

Second Point: One set of chromosomes from the egg and sperm are called sex chromosomes. The egg always has a female chromosome known as (X), while the sperm can either have a male chromosome (Y) or a female chromosome (X).

Third Point: Chromosomes are made up of many genes.

Fourth Point: Genes are either dominant or recessive. Dominant genes work alone to influence your baby's characteristics. Basically, they cancel out the instructions given by other genes.

Understanding the basics points for genetics will allow you to understand what can cause genetic disorders. Below is a review of some genetic disorders.

What is a birth defect?

Birth defects are errors in the way a baby looks or functions at birth. Birth defects can be recognized right away in the delivery room, later in the baby's life or even during pregnancy. No matter when the birth defect is recognized, all are considered a congenital or genetic disorder. Most of the abnormalities seen are mild, but some are severe and can lead to death. Some babies need surgery and others can be treated with medication. Unfortunately, in about 70% of birth defect cases, the cause is unknown.

What are the different types of genetic disorders?

Learn about the common genetic disorders and birth defects that are seen in pregnancy.

All genetic disorders are caused by problems with either the entire chromosome or with the genes on the chromosome. When a gene is passed from the parents to the child, it is called an inherited disorder. A birth defect can also be caused by a parent passing a normal chromosome to the baby that changes or mutates later, called a non-inherited disorder.

There are three types of inherited disorders:
Dominant Disorder - means there is a 50% chance that a parent who carries a bad gene will pass it to their child.

Recessive Disorder means that both parents have to carry a bad gene and both have to pass the gene to the child. The chance of this happening is about 25%.

Sex Linked Disorder is dominant and almost always is due to genes on the X chromosome.

There are countless genetic disorders. The majority of these can be detected by genetic testing with amniocentesis (the amniotic fluid surrounding the fetus is sampled and sent for tests) or Chorionic Villus Sampling (obtaining fetal cells from the placenta followed by testing on the tissue that was removed).

Parents can find out if they are carriers of most of these disorders through blood tests or other carrier tests. Here are a few genetic disorders that are seen most often:

Sickle Cell Anemia is a blood disorder that causes severe pain and anemia (low number of red blood cells). Its risk of occurrence is 1 in 600 African American babies and 1 in 1000 Hispanic babies. No current treatment is available although in a small number of cases, bone marrow transplants may offer a cure. Doctors are continuing to improve treatment and care for this disorder.

Tay - Sachs Disease causes mental retardation, blindness and seizures starting for most infants at 6 months of age. The carrier rate for this disorder is high: one in 30 Ashkenazi Jews and French Canadians. With these ancestries, one in 3000 babies are born with Tay-Sachs disease. Unfortunately, there is no cure. Children with Tay-Sachs disease usually live for only few years.

Cystic Fibrosis is a genetic disorder that mainly causes problems with breathing, which usually appear in early childhood and progressively get worse. The chance for being a carrier is high and about one in 2500 Ashkenazi Jewish babies are born with cystic fibrosis. This disorder does not have a cure and life expectance is shortened.

Thalassemia is a genetic disorder that causes anemia. There are many different types of Thalassemia disease that cause varying degrees of anemia. Depending on the degree of disease, blood transfusions may be needed throughout life.

Hemophilia is a sex linked genetic disorder. Males with Hemophilia are not able to form blood clots and when cut, bleed for extended periods of time. One in 10,000 males has Hemophilia.

Down syndrome or Trisomy 21 is a genetic disease where a baby has one extra chromosome. This causes mental retardation, heart defects, hearing problems and vision disorders. Your risk of giving birth to a baby with Down syndrome increases with advancing maternal age. One in about 800 babies in the United States is born with Down syndrome. There is no current recommended treatment.

Trisomy 18 is associated with babies that have an open spinal column or heart defect. Most babies will die in the first year of life. Trisomy 18 is caused by having one extra number 18 chromosome. This genetic disorder is seen in one out of 8000 births.

XXY or Klinefelter's Syndrome is a chromosomal genetic disorder where a boy has an extra chromosome, the X chromosome. Boys with Klinefelter's syndrome have rounded bodies and enlarged breasts, they are sterile and learn to talk later in life. About one in 1000 boy babies have an extra X chromosome and are affected by this genetic disorder.

Turner's Syndrome is the absence of one X chromosome that gives rise to a syndrome that is characterized by girls having a short stature (are less than 5 feet), start puberty late and are infertile. One in 2500 girls have the absent chromosome and are affected by this genetic disorder. Treatment with growth hormones and the sex hormones help.

What is genetic counseling?

Your OB doctor may refer you and your partner for genetic counseling. A genetic counselor will assist you in identifying the risks of having a baby with a genetic disorder, explain your options about being tested, and help you consider your options if a genetic problem is found.

During your visit with your counselor, you will be asked to answer questions about your family's medical history, have blood work done and participate in other possible diagnostic tests. After all of the testing is completed, your counselor will explain if there is an increased risk of having a baby with a genetic problem.

Most of the time, the test results reflect findings that your pregnancy and baby are normal. If the findings reveal an increased chance of having a baby with a genetic defect or disorder, you may be faced with some hard choices.

Your counselor will guide you with your decisions by educating you about any genetic problems and discussing options with you and your partner. After reviewing the problems that face you and your baby, some parents choose to terminate the pregnancy while others may decide to continue the pregnancy with the knowledge that the baby will have problems. If you choose to continue your pregnancy this time before birth can be used to plan for the challenges ahead for you and your baby.


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