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Prenatal Diagnostic Procedures
For most women, pregnancy is a healthy, happy time ending with the birth of a healthy baby. Birth defects occur in only a small percentage of pregnancies (under 5%), and prenatal screening and diagnostic procedures have been developed to help identify some of these during a pregnancy. Some couples have a higher risk of having a baby with a birth defect because of the age of the mother (40 or older at the time of delivery) or because of the family history and personal medical history of the parents (such as previous children or close relatives with genetic disorders).
These terms are used to describe prenatal diagnosis:
- Prenatal: anything that happens before birth.
- Fetus: refers to the developing baby from the 9th week of pregnancy until birth.
- Diagnosis: a doctor's opinion or belief about what is happening to your body or baby; your doctor makes a diagnosis after speaking with you, performing a physical examination, and/or ordering medical tests(s).
- Prenatal Diagnostic Testing: a variety of ways that the fetus or structures surrounding the fetus are examined during the pregnancy. (See ultrasound, amniocentesis, chorionic villus sampling)
- Ultrasound: a test that uses sound waves to get a type of picture of the baby in the uterus.
- Chromosomes: contained in every cell in the human body, and carry genes (or hereditary factors). For normal growth and development, it is important to have the correct number of chromosomes (46).
- Genes: the information in every one of your cells that determines such things as height, eye and hair colour, and how your body will form and grow. A genetic condition is a disability, disease, or quality caused by absent, mutated (changed) or abnormal genes.
- Birth Defect: any mental or physical abnormality that may begin before birth or may be found later. The most common defects are cleft lip and palate, clubfeet, heart defects, and mental retardation. Often the exact cause of the problem cannot be found and many problems cannot be identified before birth.
- Neural Tube Defect: neural tube defects occur when the brain or spinal column does not form in the usual way. Examples of neural tube defects are spina bifida, an opening in the spinal cord and column that can cause mild to severe paralysis of the lower body; or anencephaly, in which the brain does not develop normally.
- Prenatal Genetic Screening: consists of one or more blood tests, which may be done at any hospital or laboratory. Sometimes a special nuchal translucency (NT) ultrasound may be done in addition to, or instead of, the blood tests. It is not a definite diagnosis of these disorders, but can help families decide about having more testing such as amniocentesis.
- Amniotic fluid: a colourless liquid made up mostly of water, which surrounds and protects the fetus inside the amniotic sac within the uterus.
Who Should have Prenatal Diagnosis?
The decision to have prenatal diagnosis is a personal one. This is a decision that you will make after talking about your health and family medical history with your doctor or midwife. You may be offered prenatal diagnosis for the following reasons:
- Positive Prenatal Genetic Screen: If you have had this blood test and been identified as having a baby at increased risk for a chromosomal abnormality or neural tube defect.
- Advanced Maternal Age: Pregnant women who are or will be 40 years or older at the time of delivery have a greater chance of having a baby with a chromosomal abnormality. With twin/multiple pregnancies, testing is offered to women who are or will be 35 years or older at the time of delivery.
If your only extra risk factor is your age, you can get advice and counseling from your family doctor/midwife or obstetrician. If you have any extra concerns (such as a family history of genetic or other abnormalities), we recommend that your caregiver refer you to the Medical Genetics Program for counseling about prenatal diagnosis, ideally before conceiving a baby.
- Abnormal Ultrasound: a routine ultrasound test has shown a problem in your current pregnancy.
- Couples in which either person has or has had:
- Another child or stillbirth with a chromosome abnormality
- Another child with a neural tube defect such as spina bifida or anencephaly, or a close relative (brother, sister, niece, nephew) with a neural tube defect
- Another child with a known or suspected genetic disorder, birth defect, or developmental delay
- a known chromosome rearrangement
- a suspected genetic disease or birth defect.
- A family history of known or suspected genetic disorders (e.g., Duchenne Muscular Dystrophy, Hemophilia, Fragile X syndrome)
- Couples in which both partners are carriers of a genetic disorder, such as
- Thalassaemia ( more common in Mediterranean, Asian and East Indian populations)
- Tay Sachs (more common in the Ashkenazi Jewish population)
- Sickle cell anaemia (more common in the Black population)
- Cystic Fibrosis (more common in the Caucasian population)
- Exposure to certain drugs or other chemicals that may cause birth defects. This could include women who have taken Acutane, an acne drug, or those who have used cocaine or alcohol heavily during pregnancy.
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