Fertilization occurs in 4-6 hours in humans, but there are no visible signs until approximately 17-18 hours later.
The first signs of fertilization are the development of two round bodies in the center of the egg. The slightly smaller body is the female pronucleus which contains 23 chromosomes that the egg contributes to the embryo. The other round body is the male pronucleus which contains the contribution of 23 chromosomes from the sperm. The egg must be checked at this point for fertilization because, over the next 6 hours or so, the two pronuclei come together in a process known as syngamy (where the two pronuclei join chromosomes) forming one nucleus of 46 chromosomes.
Within the next 6 hours (30 hours from fertilization), the now “fertilized egg” will divide, producing a 2-cell embryo. Further division (or “cleavage”) takes place every 10-12 hours, producing a 4-cell embryo on day 2, an 8-cell embryo on day 3, a morula or ball of too many cells to count on day 4, and a blastocyst on day 5.
If the embryo is looked at too late after fertilization, then “abnormal” fertilization can be overlooked and the embryo will “appear normal.” This “abnormal” embryo can divide and even implant, but will not produce a viable pregnancy, and is only destined to abort, resulting in a miscarriage and disappointment.
