Upon release from the ovary, the oocyte (egg) undergoes a process of maturation.
Following maturation, the egg awaits fertilization by the sperm either in the fallopian tube of the female reproductive tract or, in an assisted reproduction setting, in in vitro culture media. An optimal window, fertility window, exists in which fertilization should occur; this is generally within 10 h of ovulation.
In the event that no fertilization occurs within this time frame, oocytes experience a process of degradation referred to as ‘post-ovulatory ageing’. The cellular deterioration associated with postovulatory ageing can strongly influence the outcome of fertilization in vivo and in vitro, as the oocyte provides factors to the embryo that orchestrate the early events of embryo development (embryogenesis), including repair of damages to the DNA of both egg and sperm.
Since there are no visual signs of ovulation and, as a result, no mechanisms in place to ensure synchrony between intercourse and release of the oocyte, there is an increased likelihood that fertilization could involve an aged oocyte and freshly ejaculated spermatozoa. This may result in the generation of poor quality embryos, and an increase likelihood of pregnancy loss.
To avoid late fertilization, it is recommended to try and identify your fertility window, 6 days in your menstrual cycle in which fertilization is favored.