Chromosome disorders are the most common cause of first-trimester miscarriage. Most estimates suggest that at least 50% of all miscarriages are due to chromosomal factors, with some estimates putting the number at 75% or higher. Trisomies are the most common chromosome-related finding in post-miscarriage testing. Of all trisomies, trisomy 16 seems to be the most common in humans, occurring in 1% of all pregnancies.
What Trisomy 16 Means:
Human beings are supposed to have 46 chromosomes, which are grouped together in 23 pairs. A trisomy means that an individual has three copies of a particular chromosome instead of two. Most trisomies cause health problems of varying severity. The effects can be anywhere from mild and barely noticeable to "incompatible with life," meaning all affected babies will either be miscarried or die in early infancy.
Types of Trisomy 16:
A diagnosis of full trisomy 16 would mean that all the cells in the baby's body were affected by trisomy 16. Full trisomy 16 is incompatible with life and nearly all affected babies are miscarried in the first trimester. But it is also possible to have mosaic trisomy 16, meaning some of the body's cells are affected and other cells are normal. It is also possible, in rare cases, to have a pregnancy in which the placental cells have full trisomy 16 or mosaic trisomy 16 even though the baby is chromosomally normal.
Causes of Trisomy 16:
The cause of full trisomy 16 is usually an error in cell division affecting either the sperm or the egg, meaning the abnormality is already present at conception. Mosaic trisomy 16 in either the baby or the placenta usually results from the trisomy present at conception "correcting" during cell division very early in fetal development, leaving some cells affected but not others. Researchers have not determined what causes these cell division errors but they appear to occur at random.
Full trisomy 16 may be diagnosed as the cause of a miscarriage if parents collect tissue and request chromosomal testing after the loss, but trisomy 16 can also be diagnosed during pregnancy through CVS or amniocentesis.
What the Diagnosis Means:
Full trisomy 16 nearly always results in first-trimester miscarriage. If you have been told that trisomy 16 was the cause of your miscarriage, you should know that the miscarriage was not your fault and the odds are low that your next pregnancy will be affected.
If you are currently pregnant and have received CVS or amniocentesis results showing cells affected by trisomy 16, you are most likely concerned and confused, and that's completely normal and okay. You should know, however, that it's highly unlikely that the baby has full trisomy 16 if your pregnancy has progressed thus far -- the baby may have mosaic trisomy 16 or the trisomy may be confined to the placenta. Not a lot of research exists on outcomes for either condition, but the research that is available suggests that affected babies usually will not suffer serious complications. There is a higher risk of preterm birth and intrauterine growth restriction, but beyond pregnancy, the health effects can vary widely. Sometimes babies are born with no evidence of any abnormalities but others can have a set of characteristic health problems, depending on the degree of the mosaicism. In some cases, chromosome 16 disorders can also include cognitive impairments and developmental delays. But in most cases it appears that the baby will be developmentally normal. Unfortunately, it is not possible to predict the specifics of the outcome for the baby based on the CVS or amniocentesis results.
Regardless of any studies or predictions, it is normal to be upset and to have questions whenever any test shows that there may be any problems with your baby. So it may be helpful to speak with a genetic counselor about what to expect and to get answers to any questions you may have.
Frequently Asked Questions About Chromosome 16 Disorders. Disorders of Chromosome 16 Foundation. Accessed: Mar. 15, 2009. http://www.trisomy16.org/html/faqsdisorders.html.
Trisomy 16 mosaicism. University of British Columbia. Accessed: Mar. 16, 2009. http://www.medgen.ubc.ca/robinsonlab/mosaic.htm.