During fertilization, a sperm cell penetrates a woman's egg and the egg becomes a zygote, notes HowStuffWorks. The zygote experiences rapid cell growth and turns into a blastocyst, before traveling down the fallopian tube and connecting itself to the uterine wall in a process known as implantation.
Conception begins when a female egg is fertilized by sperm, and once this occurs, the resulting zygote has DNA information from both the egg and the sperm, states HowStuffWorks. The cells in the zygote begin to multiply rapidly about 24 hours after its formation, and they form a solid cluster of cells. These cells then turn into a hollow blastocyst, with cells on the inside that turn into an embryo and an outer wall that turns into the placenta and other tissues necessary to fetal development. The blastocyst travels down the fallopian tube in about four days and makes its way into the uterus approximately 24 hours later.
After entering the uterus, the mass of cells inside a blastocyst comes out of the outer membrane in about 24 hours, according to HowStuffWorks. Implantation occurs when the blastocyst and the uterine wall, or endometrium, exchange hormones, and the blastocyst adheres to the wall. For about 48 hours after implantation, women may experience light bleeding, or spotting. Within about three weeks of implantation, the first nerve cells in the fetus begin to form.