Menstrual Cycle and Fertilization

Phases of the Menstrual Cycle

• Menstrual Phase (Menses) occurs on days 1-5, involving the shedding of 50-150 mL of blood, mucus, and endometrial tissue.
• Preovulatory Phase takes place on days 6-13, marking the period between menstruation and ovulation.
• Ovulation occurs on day 14 as the Graafian follicle ruptures, which releases the secondary oocyte into the pelvic cavity.
• Postovulatory Phase spans days 15-28, representing the time between ovulation and the next menstruation.

Stages of Development: Fertilization

• It is the union of sperm and ovum in the outer 1/3 of the fallopian tube, which forms a zygote thereby restoring 46 chromosomes.
• Dizygotic (Fraternal) Twins are two separate oocytes fertilized by different sperm and they may be the same or different sex.
• Monozygotic (Identical) Twins are a single fertilized ovum splits into two embryos with the same genetic material and always the same sex.
• Conjoined Twins occur when late separation (after 8 days) takes place, resulting in twins sharing body structures.

Stages of Development: Implantation

• The egg is free in the uterus for -2 days before attaching to the endometrium (6-10 days after fertilization).
• Human chorionic gonadotropin (hCG) is detected in maternal blood, forming the basis of early pregnancy tests (urine tests are less sensitive).

Stages of Development: Embryonic Stage

• The zygote undergoes cleavage (mitosis) within 72 hours, forming a 16-cell morula.
• The blastocyst inner cells develop into the embryo and amnion.
• Trophoblast outer cells form the chorion and placenta, producing hCG to maintain the corpus luteum, which secretes progesterone.

Stages of Development: Fetal Period

• Fetal Period takes place from week 9 to birth.
• Growth and differentiation of tissues and organs formed in the embryonic stage occur during this time.
• Few new structures develop, but rapid body growth occurs, especially in the second half of pregnancy.
• The fetus becomes less vulnerable to drugs, radiation, and infections compared to the embryonic stage.

Main Idea

• The structures of the human reproductive systems are specialized for the production of offspring.
• Males produce sperm in their testes continuously throughout their lifetime.
• Female ovaries produce eggs, and egg production begins before a female is born.
• Females have a limited number of eggs and reproductive years.
• Sperm swim through a female's reproductive tract to reach an egg.
• Fertilization usually happens in a female's fallopian tubes, where only one sperm usually fertilizes an egg.
• Fertilization produces a zygote that develops into an embryo and implants into the uterus.
• The normal development of a fetus depends on the good health of its mother.
• Pregnancy usually lasts about 38 weeks.
• The organs and major structures of the fetus begin forming during the first trimester.
• A fetus depends on its mother for nutrition and waste removal.
• A mother's prenatal care and lifestyle choices affect the health of her fetus.