What is fetal karyotyping?

Karyotyping is the examination of a baby's chromosomes under a microscope, to count them and check their structure. It is performed on cells obtained by amniocentesis or CVS. The cells are cultured so that dividing chromosomes can be photographed and arranged in pairs — which is where the characteristic image of paired chromosomes comes from. What it detects: extra or missing whole chromosomes, such as trisomy 21; large structural changes including translocations and inversions; and mosaicism, where different cells have different chromosomes. What it does not detect: small deletions and duplications below the resolution of a microscope. Chromosomal microarray is used for those, and it is often performed alongside or instead. Where karyotyping retains an advantage over microarray: it can identify balanced rearrangements, such as a balanced translocation, which microarray cannot detect because no material is gained or lost. That matters when investigating recurrent miscarriage, because a balanced translocation in a parent is exactly what you might be looking for. Because it requires cell culture, karyotyping takes longer than microarray. Cells occasionally fail to grow, requiring a repeat sample. The same technique is used on blood samples to test adults — this is what is meant by parental karyotyping after recurrent miscarriage, or karyotype testing in men with very low sperm counts.