The morning after egg retrieval and the introduction of the egg to sperm, an embryologist observes the eggs and records their fertilization status. The following photographs reveal what the embryologist may see and the factors they examine when evaluating fertilization. This is a mature, unfertilized egg. The presence of a polar body (circled in red) indicates that the egg was mature and ready to be fertilized. The absence of pronuclei and a 2nd polar body indicates that the egg failed to fertilize. Note the single sperm (arrow) attached to the outside of the eggshell. This is a normally fertilized egg with two pronuclei (circled in blue) and two polar bodies (circled in red). This is an abnormally fertilized egg with three pronuclei (circled in blue). In this case, because there is only one polar body, (circled in red), we would suspect that there are 2 maternal pronuclei in addition to the paternal pronucleus. This is an abnormally fertilized egg with only one pronucleus, (circled in blue) and one polar body (circled in red). This is an abnormally fertilized egg with four pronuclei (circled in blue).