During a typical menstrual cycle, one egg is released from one of the ovaries around day 14. This process is called ovulation. The released egg then enters the funnel-shaped end of one of the fallopian tubes. During ovulation, the mucus in the cervix becomes more elastic and fluid, allowing the sperm to enter the uterus rapidly.
Once the sperm enters the uterus, they can travel through the cervix and into the fallopian tubes, where fertilization occurs. The lining of the fallopian tube facilitates fertilization.
If fertilization does not occur, the egg continues to move down the fallopian tube towards the uterus, and it eventually degenerates and passes through the uterus with the next menstrual period.
However, if the sperm penetrates the egg, fertilization occurs, and the resulting zygote moves down the fallopian tube towards the uterus. Cilia lining the fallopian tube help to propel the fertilized egg towards the uterus.
As the fertilized egg moves down the fallopian tube, its cells divide repeatedly, forming a ball of cells called a blastocyst. About 6 days after fertilization, the blastocyst implants in the wall of the uterus.
If more than one egg is released and fertilized, the resulting pregnancy involves more than one fetus, usually two. These are fraternal twins, as each fertilized egg is genetically different.
In the case of identical twins, one fertilized egg separates into two embryos after it has begun to divide. Since one egg was fertilized by one sperm, the genetic material in the two embryos is identical.
Overall, the process of fertilization and implantation is a complex but crucial part of human reproduction.