The Reproductive System and Stages of Human Development

Reproductive System Reproduction begins with sex hormones that signal to the body to begin producing gametes The body begins releasing sex hormones during puberty When a person reaches puberty, their pituitary gland (which is found in the brain) will release testosterone/estrogen/progesterone into the blood stream

In women the variation of the levels of each hormone results in the stages of the menstrual cycle This cycle lasts about 28 days and the purpose is to prepare the body for the possibility of pregnancy

The Menstrual Cycle Day Description 1-5 6-12 13-18 19-28 Menstruation: the shedding of the endometrial lining 6-12 Uterus lining starts to thicken, the follicle begins developing in the ovary 13-18 Ovulation, the optimal time for fertilization 19-28 The ovum (fertilized or not) travels down the fallopian tube until it reaches the uterus. The endometrial lining continues to thicken until the ovum reaches the uterus. If fertilization has occurred the embryo will attach to the lining, if not the uterus will prepare to shed the lining during menstruation

Pregnancy Once the sperm are in the vagina, they swim toward the uterus, then the fallopian tubes Several million sperm are released, a few thousand will reach the ovum, only one will succeed in fertilization

After fertilization the zygote moves from the fallopian tube to the uterus As it travels there, the zygote undergoes cell divisions- by the time it reaches the uterus it is a group of about 16 cells The inner most cells will form the embryo, and the outer cells the placenta Around the 14th day of development the embryo, amniotic fluid that protects it and the placenta have formed

During pregnancy the following stages occur: Zygote (fertilization – 2 weeks) Embryo (2 weeks – 8 weeks) Fetus (8 weeks – Birth)