Fertilization usually occurs in the mid-portion of the Fallopian tube. Out of the many million sperms released simultaneously, only one sperm penetrates the ovum. Immediately a barrier is formed around it that normally prevents other sperms from entering it. The developing embryo, now called Blastocyst, moves down the tube into the uterus. Once in contact with the endometrium of the uterus, the Blastocyst gets surrounded by an outer layer of a multinucleate mass, which is the Syncytiotrophoblast with no discernible cell boundaries and an inner layer consisting of individual cells, the Trophoblast proper. In this way the seed of the future baby is born.
1.2.4 Conception & Implantation
This refers to the attachment of the Zygote (the fertilized ovum) to the wall of the uterus. The Zygote generally gets implanted at the top of the uterus, beginning between 5 and 8 days after fertilization. This process is completed by 9 or 10 days during the post-conception. At this time the outer layer of this cell mass or Trophoblast, attaches itself by secreting the required enzymes, which actually erode the uterine wall cells. However, in certain rare cases the fertilized egg may develop outside the uterus. This is Known as Ectopic pregnancy.