March 03, 2011 12:00 AM
I am pleased to dedicate this month’s column to the subject of down syndrome, specifically as it relates to obstetrics. there are several ways to screen for and diagnose down syndrome — blood tests, ultrasounds and more invasive testing — which I will review in some detail.
Down syndrome is named after Dr. John Langdon down, the British doctor who first identified the condition in 1862.
We have learned a substantial amount about down syndrome, just over the past 20 years, about testing, caring for those affected and about ourselves.
Traits are passed from each parent through genes and chromosomes. each cell in our body is made up of genes and chromosomes. each chromosome carries many genes. Half of the fetus’ genes comes from the mother, half from the father. Human cells normally contain 23 pairs of chromosomes; one chromosome in each pair comes from your father, the other from your mother.
Down syndrome is caused by the presence of an extra copy of chromosome 21 in the cells of the developing baby. there are three types of cell division involving chromosome 21. all three result in extra genetic material from that chromosome, which is responsible for the characteristic features and development of down syndrome. The three genetic variations that can cause down syndrome include:
- Trisomy 21. more than 90 percent of cases are caused by trisomy 21. An individual with trisomy 21 has three copies of chromosome 21 — instead of the usual two copies — in all of his/her cells. this form of down syndrome is caused by the abnormal cell division during the development of the sperm or egg cell.
- Mosaic down syndrome. in this rare form, individuals have some cells with an extra copy of chromosome 21. this mosaic of normal and abnormal cells is caused by abnormal cell division after fertilization.
- Translocation down syndrome. in this form, part of chromosome 21 becomes attached (translocated) to another chromosome, before or at conception. Individuals with translocation down syndrome have the usual two copies of chromosome 21, but they also have additional chromosome 21 attached to the translocated chromosome. this form of down syndrome is uncommon.
Most cases are not inherited. They are caused by a problem with cell division during development of the egg, sperm or embryo.
Translocation down syndrome is the only form of this condition that can be passed from parent to child. however, only about 4 percent of individuals with down syndrome have translocation and only half of these cases are inherited from one of the parents.
In these translocation cases, the mother or father is a balanced carrier of the translocation, which means he or she has some rearranged genetic material, but no extra-genetic material. A balanced carrier has no signs or symptoms of down syndrome, but he or she may pass the translocation on to children.
Some parents have a greater risk of having a baby with down syndrome. Risk factors include:
- Advanced maternal age. A woman’s chances of giving birth to a child with down syndrome increase with age because “older” eggs have a greater risk of improper chromosome division. by age 35, a woman’s risk of conceiving a child with down syndrome is 1 in 400. by age 45, the risk is 1 in 35. however, most children with down syndrome are actually born to women under age 35.
- Having had a child with down syndrome. Typically, a woman who has one child with down syndrome has about a 1 percent chance of having another child with down syndrome.
- Being carriers of the genetic translocation. Both men and women can pass the genetic translocation for down syndrome on to their children.
In an unscreened population, about 1 in 700 babies is born with down syndrome. Early screening for down syndrome — to determine whether there is a high risk that the developing fetus has the condition — is offered as a routine part of prenatal care, as recommended by the American College of Obstetricians and Gynecologists, regardless of maternal age. The most sensitive noninvasive screening available today is the “Integrated Test.”
The Integrated Test is performed in two stages. The first test is done between 10 and 13 weeks of pregnancy. The second stage is performed at 15 or 16 and no later than 22 weeks.
The first stage involves an ultrasound examination to measure a space at the back of the baby’s neck called the nuchal translucency (NT) thickness; higher than average NT is a risk for down syndrome. Additionally, a blood sample is taken to measure the concentration of the pregnancy-associated plasma protein-A marker; low levels of PAPP-A are a risk factor for trisomy 21.
The second stage involves taking a blood sample to measure the concentration of the following four markers: alpha-fetoprotein (AFP), human chorionic gonadotropin (HCG), unconjugated estriol and inhibin-A. The NT measurement and levels of the five markers in your blood are used, together with your age, to estimate your risk of having a pregnancy affected by down syndrome.
The results are usually ready within one week after the second blood sample is taken. Because screening, as opposed to diagnostic, testing has a greater chance of being wrong, most women with “screen positive results” will not have a pregnancy with down syndrome. About 90 percent of screen positive results are, in fact, false positives. But those women whose various results place them at high risk will be offered more precise — if invasive — testing.
Amniocentesis is usually performed between 14 and 20 weeks of pregnancy. It is a procedure performed by a qualified obstetrician during which a small sample of amniotic fluid is obtained by inserting a fine needle through the abdominal wall of the mother and into the uterus. The sample of fluid is sent to the laboratory for diagnosis of chromosomal anomalies such as down syndrome. An amniocentesis is an invasive procedure, which means there is a small risk of miscarriage (usually 1 in 200) associated with it.
Results of amniocentesis, which has a very high level of accuracy, usually take one to two weeks.
A hot topic in screening research is that a blood sample taken from the mother may actually detect small amounts of the baby’s DNA genetic material, which can then be tested. this testing may accurately analyze the baby’s chromosomal makeup and determine such genetic problems as down syndrome without subjecting the mother to invasive testing such as amniocentesis.
This research is currently under way worldwide with hopes of introducing this cutting-edge technology into prenatal testing soon to high-risk patients.
A genetic counselor will discuss your baby’s diagnosis in detail with you and your obstetrician. Prenatal education and support should be facilitated by specialists with arrangements for appropriate medical services soon after delivery. in addition to local agencies such as the Kennedy-Donovan Center in new Bedford and the Schwartz Center for Children in Dartmouth, specialized services are available at Brown University School of Medicine’s Hasbro Children’s Hospital in Providence and Boston Children’s Hospital.
The down Syndrome Program at Boston Children’s Hospital offers specialized services for all children with down syndrome and their families. The staff works closely with children, parents, medical specialists, community physicians and educators. The program serves children of all ages and provides families who receive a prenatal diagnosis information and support as needed.
Weekly clinics with inter?disciplinary services for children are offered during the important period from birth to 3 years of age. The program focuses on medical and developmental monitoring. The family and child meet every six months with a team composed of a developmental pediatrician, physical therapist, nutritionist, speech pathologist, dentist, audiologist and the program coordinator, who is also a parent of a child with down syndrome. The program coordinator distributes information about state organizations, local parent groups and community resources.
I close with a quote from a wonderful patient of mine:
“Childbirth evokes a wide range of emotions, and learning about our child having down syndrome raised concerns and questions. Yet, my husband and I have learned that raising a child with down syndrome is more typical than not. Our son, Thomas, is a delightful, beautiful child who has enriched our lives beyond our expectations.”
Dr. Christian S. Pope specializes in obstetrics and gynecology. He has offices in new Bedford and Mattapoisett and practices at St. Luke’s Hospital, the new Bedford site of Southcoast Hospitals Group. He can be reached at 508-999-6245.
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