This is an update of previous 2001 Canadian guidelines on how doctors and diagnostic lab specialists should use and interpret a technology called chromosomal microarray analysis (CMA) when this test is done on human DNA during an ongoing pregnancy. The authors developed these guidelines after reading and interpreting up-to-date international literature in the field, and discussing recommendations specific to the Canadian health care system. CMA should be offered after expert counselling of the family, in situations when birth defects are present and when an entire extra or missing chromosome is not likely to be the cause of these birth defects. (By Dr. W.T. Gibson, http://jmg.bmj.com/content/early/2018/03/01/jmedgenet-2017-105013 )