Trisomy 16 occurs when an individual has three copies of chromosome 16 instead of two copies (one from each parent.) For a brief layman's review of how trisomy occurs you can check out my overview post here.
Like other trisomies, there is full trisomy 16 and mosaic trisomy 16. In full trisomy 16 all of the body's cells have an extra copy of chromosome 16. This form is not compatible with life and causes miscarriage It is the most common chromosomal cause of miscarriage in the first trimester. In the mosaic form the effects are less severe and can vary widely. Symptoms of mosaic trisomy 16 include slow growth before birth (intrauterine growth retardation, IUGR), delayed development, heart defects, speech delays, kidney defects, and reproductive disorders. There is also a partial trisomy 16 (16p+ or 16q+) where there is an extra arm of chromosome 16. Interestingly, one source (NIH) I looked at indicated that one form of duplication (16p11.2+) may give rise to an autism spectrum disorder and language delay.
Trisomy 16 disorders can be discovered prenatally. I say discovered because often the tests cannot completely determine the level of trisomy 16 that is present. Alpha Fetal Protein (AFP) screening, chorionic villus sampling (CVS) and amniocentesis are all tests that can be performed, but each has its limitations and risks. It is important to discuss any test results with a genetic counselor. Amniocentesis is probably the most accurate diagnostic procedure. Ultrasounds can help determine whether or not any physical defects are present. Many of these can be corrected by surgery. It is sometimes possible that the trisomy is confined to placental tissues (which can lead to premature birth and/or hypertension in the mother). A pregnancy that continues beyond the first trimester would indicate mosaic trisomy at most.
I would strongly encourage you to read some stories from parents of children born with mosaic trisomy to learn more from their personal experiences.
For more information please see the following links: