But first……….what are chromosomes?
Meiosis is a special type of cell division that occurs in our sex cells (eggs in females, sperm in males). This cell division results in egg or sperm that contain only 23 chromosomes (called a gamete). At the time of fertilization, the egg & sperm come together to form a cell (called a zygote) that typically has 46 chromosomes. This zygote may go on to form an embryo.
Sometimes this process of cell division does not happen properly and a developing embryo can have too many chromosomes (such as a trisomy – 3 copies of a particular chromosome instead of 2 copies) or too few chromosomes (monosomy – 1 copy of a particular chromosome). Having too many or too few chromosomes can lead to problems with how our bodies grow and develop, and can sometimes lead to spontaneous pregnancy losses.
What chromosome changes can prenatal testing tell me about?
Babies and children with Down syndrome are individuals who are just as unique and highly variable as anyone else. An individual with Down syndrome has an extra copy of chromosome 21 (trisomy 21). This usually causes mild to moderate intellectual disability, which means that adults will typically function at the level of an 8-10 year old. An individual with Down syndrome is expected to learn to read and write and to be physically active, however major developmental milestones will be delayed. Individuals with Down syndrome have a greater possibility of health conditions than the average person, such as heart, stomach, bowel, thyroid, vision and hearing problems. Treatment is available for many of these conditions. There is no way to predict how serious or mild these differences will be. People with Down syndrome generally live into their fifties. In general, about one in 1000 babies born has Down syndrome. The chance of having a child with Down syndrome increases with the pregnant person/egg donor’s (in the case of a donated egg) age (See Table below for specific information). For more information on Down syndrome, visit the Canadian Down Syndrome Society website.
Trisomy 18 (Edward syndrome)
Babies with trisomy 18 have an extra chromosome 18. Every individual with trisomy 18 is different, however serious intellectual disabilities and congenital anomalies that may affect many organ systems (e.g. heart, kidneys) are expected. Many pregnancies with trisomy 18 will miscarry. Most babies born with trisomy 18 do not survive past the first few months of life. About 10% of infants will survive up to 5 years of age, or occasionally longer. Long-term survivors are described as socially interactive with significant physical and intellectual disabilities (e.g. having few words). In general, about one in 6,000 babies born has trisomy 18. The chance of having a child with trisomy 18 increases with the pregnant person/egg donor’s age. For more on trisomy 18, visit The Trisomy 18 Foundation website.
Additional conditions which may be screened for using prenatal screening:
Trisomy 13 (Patau syndrome)
Babies with trisomy 13 have an extra chromosome 13. In general between 1/8,000 and 1/15,000 of babies born has trisomy 13. The chance of having a child with trisomy 13 increases with a pregnant person/egg donor’s age. Every individual with trisomy 13 is different; however each individual is expected to have both serious intellectual disabilities and congenital anomalies, including differences of the brain, heart, eyes and face. Most pregnancies with trisomy 13 will miscarry. About 5-10% of infants will survive the first year of life. There are some individuals who have lived to adulthood. For information on trisomy 13, visit Support Organization for Trisomy 18, 13 and Related Disorders (SOFT) website.
Sex chromosome differences
Typically, each mature human cell contains 23 chromosome pairs (46 chromosomes in total). One of these chromosome pairs, the 23rd pair, is the sex chromosomes. A female cell will usually have two X chromosomes and a male cell will have one X chromosome and one Y chromosome. Some prenatal screening and diagnostic testing can identify extra or missing X and/or Y chromosomes. There are several disorders with sex chromosome differences. Generally, these types of genetic differences are quite common occurring in about 1 in 500 individuals. Expected features are variable and depend on which chromosome is extra or missing. Some features may be as mild as tall or short stature. There may be differences in development such as delayed speech or learning differences. There may be associated health issues involving puberty or the heart. For a brief overview of chromosomes and some chromosome disorder visit the Genetics Home Reference
Chance of Chromosome Abnormalities in Liveborn Infants at Term by Pregnant Person’s Age
Pregnant Person/ Donor’s Age (yrs)
Chance of Down Syndrome
Chance of ANY Chromosome Difference