Prenatal development begins with fertilization, when a sperm cell joins an egg cell through unprotected sexual intercourse. Conception takes place after the strongest sperm cell travels through the female’s fallopian tubes to meet the egg cell and form a zygote. The fertilized egg, or zygote, eventually divides into a blastocyst, an inner group of cells with an outer shell. The blastocyst, its inner layer being an embryo and outer layer being membranes that support the embryo, implants itself into the uterus and receives nourishment from the mother's uterine wall blood lining. Five weeks after conception, major organs systems and structures being to develop during the embryonic period.Around the eleventh to thirteenth week, the embryo becomes enters into its second trimester and becomes a fetus. The baby’s bones and muscle tissue begin to form, allowing movement. The intestinal tract produces the baby’s first bowel movement. By the end of the second trimester, the baby’s heartbeat can be heard through a stethoscope and the fetus gains abilities such as hearing and may respond to loud noises. Features such as minimal head hair, fingernail, eyebrows and eyelashes begin growing during this phase.