Zona Pellucida Royalty-Free Stock Photo

The thick transparent membrane surrounding a mammalian ovum before implantation. When trying to conceive, it's sometimes hard not to scrutinize every sign and symptom you feel for possible signs of pregnancy. One of the most tell tale signs of pregnancy for many women is implantation, or when a fertilized egg, or zygote, implants into the lining of the uterus. Implantation typically occurs 6 to 10 days after ovulation, or day 20 to 24 of a menstrual cycle. However, it can happen a few days earlier or later. In an average 28-day cycle, ovulation occurs on day 14 with the first day of a woman's period being considered day 1 in a cycle. Once fertilization occurs, the egg stays in the fallopian tube for three days before entering the uterus and begins to implant itself to the uterine lining.