The first trimester is crucial for the development of the various organs, tissues, and muscles. The second trimester is a period of rapid growth. In the third trimester, there is fat accumulation and consolidation of growth and development. The full-term fetus is born at the end of the third trimester.The miracle of life begins with the momentous encounter between the male and female gametes, each with half the chromosomal content of a normal human cell. The mature ovum or egg is released by the ovary in the female reproductive system, while the sperm is deposited in the vagina by the male during intercourse. Millions of tadpole-like sperm, with heads and tails, swim to the fallopian tube, where the ovum waits, facilitated by the secretion of a white, slippery mucous lining by the uterus. One sperm penetrates the thick outer covering, or the zona pellucida, of the ovum with its’ head, by the secretion of an enzyme acrosin. Immediately, a chemical reaction makes the zona pellucida impenetrable to other sperm. The tail of the successful sperm detaches, leaving only the head within the ovum. This ovum with the sperm head, or the fertilized egg, is called the conceptus. Conception, as a rule, occurs in the outer third of the fallopian tube. The genetic material of the ovum and the sperm fuses, restoring the chromosomal number to 46, creating a single-celled zygote. The ovum has only the X chromosome, while the sperm carries either the X or the Y chromosome. If the ovum is fertilized by an X chromosome carrying sperm, the resulting baby will be a female. if the ovum is fertilized by a Y chromosome carrying sperm, a baby boy will be the result.The zygote travels down the fallopian tube and moves into the uterus, where it floats. Within two days of zygote formation, cleavage of the zygote begins, in which it divides into two cells or blastomeres. As there is no synthesis of cytoplasm, the divided cell volume decreases. Further cell division of the zygote leads to the formation of a sixteen celled solid ball of cells, called the morula, with a mulberry-like appearance, which is 1/1000 inch wide.