Understanding Chromosome Conditions and Birth Defects

What are chromosomes?

Our bodies are made up of billions of cells. Each cell contains genetic material that is inherited from our parents and stored in structures called chromosomes. Our chromosomes contain the instructions that tell our bodies how to grow, develop, and function. Normally we have 46 chromosomes in each cell (see Figure 1 below). One set of twenty-three chromosomes comes from the egg of our mother and one set of 23 comes from the sperm of our father. The first 22 pairs of chromosomes are the same in men and women. The last pair, the sex chromosomes, determines gender. Females have two X chromosomes and males have one X and one Y.

What is a chromosome condition?

A chromosome condition occurs when there are extra or missing chromosomes, or a rearrangement of pieces of chromosomes. Intellectual and physical development can be affected as a result of this chromosome difference. Down syndrome, also known as trisomy 21, is the most common chromosome condition. People with Down syndrome have 47 instead of 46 chromosomes, having an extra chromosome 21.

Babies can be born with other chromosome conditions. Trisomy 13 or trisomy 18 are two chromosome conditions that occur when there is an extra chromosome 13 or chromosome 18. Both of these conditions cause severe physical and intellectual handicaps. Differences can also occur with the sex chromosomes (X or Y). These are usually associated with milder problems, if any. There are many other chromosome conditions that can occur, some of these have no or very minor impact on the person, others can cause severe physical and intellectual handicaps.

Figure 1:
Two microscope images of a female cell with 46 chromosomes (left) and a male cell with 46 chromosomes (right) showing the standard arrangement in matched chromosome pairs called a ‘karyotype’.

Who is at risk for having a baby with a chromosome condition?

A baby with Down syndrome or other chromosome conditions can be born to any couple. What is known is that the risk increases as a woman ages. Regardless of a woman's age however, the chance of having a healthy baby are always in your favor.

Maternal Age and Chromosome Conditions

The risk of having a child with a chromosome condition, such as Down syndrome, trisomy 13 and trisomy 18, increases slightly as a woman ages (Table 1). Because of this, women 35 years or older are offered the option of prenatal diagnostic testing (with amniocentesis or chorionic villus sampling). These tests diagnose (gives a “yes” or “no” answer about) chromosome conditions.

Why is 35 the age when prenatal diagnostic testing is offered?

There is a balance of risks and benefits when a woman reaches 35 years of age, so the current policy is to provide women with this risk / benefit information to assist them in making their own decision about testing.

To explain, amniocentesis or chorionic villus sampling place a pregnancy at a 1/200 (0.5 percent) increased risk for miscarriage. As you can see in the second column of Table 1, the chance of detecting a chromosome condition for a 35 year old is 1/250 and for a 36 year old is 1/200. (You will find in this table that the chromosome risks in “mid” pregnancy are higher then from “term” because many babies with chromosome conditions are more likely to miscarry during pregnancy.) This age is where there is roughly a balance between risk and benefits.

Of note - the chance of having a baby without a chromosome condition are always in a woman's favor. For example, a 35 year old woman has a 99.5% chance at term that her baby would not have a chromosome condition. A 42 year woman has a 97.5% chance of a baby with normal chromosomes.

What is a birth defect?

A birth defect is a physical condition that a baby is born with. Examples of birth defects include spina bifida, heart defects and cleft lip.

What are my chances for having a baby with a birth defect?

All women, regardless of their age and family or previous pregnancy history, have a 2–3 percent chance of having a baby born with a birth defect. Many of these defects can be corrected and have no impact on the child’s overall health. However, some birth defects can be life-threatening or result in lifelong physical and/or intellectual handicap.

What are open neural tube defects?

The neural tube is a part of the developing baby that forms very early in development. The upper part of the neural tube forms the baby’s brain and the lower part forms the spine. If the neural tube does not form properly the baby will have a neural tube defect (NTD). Spina bifida is an example of a neural tube defect. The physical and intellectual effects of a neural tube defect vary greatly from person to person.
Spina bifida may be fairly mild, or can be more serious, affecting the nerves of the bladder, bowel and legs.

What are my chances for having a baby with a neural tube defect?

When there is no family history, the risk of a having a child with a NTD, regardless of a woman’s age, is about 1 in 1500 in southern Alberta. If there is a family history of neural tube defects, this risk may be slightly higher.

What about folic acid and neural tube defects?

The number of babies born with neural tube defects has been declining since folic acid (a type of vitamin B) was added to flour and cereal grain products in Canada.

To ensure women are receiving enough folic acid in their daily diet, it is recommended that ALL women considering pregnancy to take a supplement of folic acid by adding 0.4 mg of folic acid to their diet daily. This can be obtained by taking a multivitamin tablet (read the back of the label of the multivitamin to find how much folic acid is in the tablet). Women should begin folic acid at least one month before pregnancy and continue through the first trimester.

What testing options are available to me in my pregnancy?

The testing available depends on the stage of the pregnancy and the woman’s risk factors. This website outlines the common screening and diagnostic testing options available.

Where can I go to learn more about my testing options?

The Calgary Prenatal Genetics Clinic provides a regularly scheduled Prenatal Testing Options information class Appointments can be made by calling (403) 943-8375.

What should I know about my family history?

• genetic conditions
• their family history and what that means to their pregnancy and family
• genetic testing and whether genetic testing meets their personal needs
• results of genetic testing and what that means to their pregnancy and family
• results of screening ests and what that means to their pregnancy and family

If you are concerned about your family history you are welcome to contact the Prenatal Genetics Clinic in Calgary at (403) 943-8375.