Combining Chromosomes is like making a recipe if you add too much of an ingredient the recipe could fail or not taste good.
Trisomy 18 is where too much of an "ingredient" is added, or an extra chromosome in this case
When the Sperm meets the egg an additional Chromosome is created this can lead to birth defects, most likely a miscarriage or fetal death, and possibly infant death later.
Trisomy 18 is completely accidental, it happens about 1 in every 3,000-6,000 live births. It is more likely to happen is the mother is aged.
Down syndrome is also caused by an extra chromosome
Bibliography:"Edwards' Syndrome." NHS Choices. NHS, n.d. Web. 22 Mar. 2017.