Trisomy 16 and First-Trimester Miscarriages

A common cause of first-trimester miscarriages

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Chromosomal disorders are the most common cause of first-trimester miscarriages. Estimates suggest at least 50 percent of all miscarriages are due to chromosomal factors, with some estimates going as high as 75 percent 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, occurring in approximately 1 percent of all pregnancies.

Normally, people have 46 chromosomes, which are grouped together in 23 pairs. Having a trisomy means that an individual has three copies of a particular chromosome instead of two. Many trisomies cause significant health problems while others can be fatal. Trisomy 16 is considered a fatal trisomy, often causing first-trimester miscarriages.

Types of Trisomy 16

There are three types of trisomy: full, partial, and mosaic. Full trisomy 16 means that all of the cells in the baby's body are affected. Full trisomy 16 is incompatible with life and nearly all babies who inherit the condition are miscarried in the first trimester. It is also possible to have mosaic trisomy 16, meaning some of the body's cells are affected while 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 –– known as partial trisomy 16.

Full trisomy 16 is usually caused by a cell division error that affects either the sperm or the egg, meaning the abnormality is present at conception. Mosaic trisomy 16 in either the baby or the placenta usually results from the trisomy trying to correct itself during cell division very early in fetal development, leaving some cells affected but not others.

Researchers have yet to determine 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 chorionic villus sampling (CVS) or amniocentesis.

What a Trisomy 16 Diagnosis Means

Full trisomy 16 almost 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, it's completely normal to be scared or confused. You should know, however, that it's unlikely that the baby has full trisomy 16 if your pregnancy has progressed past the first trimester –– 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 do not suffer serious complications.

There is a higher risk of preterm birth and intrauterine growth restriction. Beyond pregnancy, the health effects can vary widely. Sometimes babies are born with no evidence of any abnormalities while 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. Unfortunately, it is not possible to predict the outcome based on the CVS or amniocentesis results.

Regardless of any studies or predictions, it is normal to be upset and have questions when testing shows there may be problems with your baby.

You can speak with a genetic counselor about what to expect as well as ask any questions you may have.


Frequently Asked Questions About Chromosome 16 Disorders. Disorders of Chromosome 16 Foundation. 

Trisomy 16 mosaicism. University of British Columbia. 

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