12.4 Fertilization, pregnancy, and parturition

Fertilization, pregnancy, and birth are crucial stages in human reproduction. Sperm undergo capacitation and the acrosome reaction to fertilize an egg, forming a zygote. This process typically occurs in the fallopian tube, leading to implantation in the uterus.

Pregnancy involves embryo and fetal development, supported by the placenta. Gestation lasts about 40 weeks, divided into trimesters. Labor and delivery mark the end of pregnancy, followed by postpartum changes and lactation to nourish the newborn.

Fertilization

Sperm Preparation and Fusion

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• Capacitation enables sperm to fertilize an egg by removing glycoproteins and altering membrane composition, which increases sperm motility and prepares them for the acrosome reaction
• Acrosome reaction occurs when a capacitated sperm comes in contact with the zona pellucida of an egg, causing the release of enzymes from the acrosomal vesicle that digest the egg's outer layer and allow the sperm to penetrate
• Zygote formation happens when a sperm successfully fertilizes an egg by fusing with its plasma membrane and combining genetic material, creating a diploid cell that will develop into an embryo

Fertilization Process and Location

• Fertilization typically occurs in the ampulla of the fallopian tube, where the egg and sperm meet after ovulation
• Sperm can survive in the female reproductive tract for several days, waiting for an egg to be released during ovulation
• Once a single sperm penetrates the egg's zona pellucida and plasma membrane, the egg undergoes changes that prevent additional sperm from entering (cortical reaction)

Pregnancy

Implantation and Early Development

• Implantation occurs when the blastocyst (early embryo) attaches to the endometrium about 6-10 days after fertilization
• Trophoblast cells from the blastocyst invade the endometrium and later develop into the fetal portion of the placenta
• Embryo refers to the developing human during the first 8 weeks of pregnancy, during which time major organ systems begin to form

Placenta and Fetal Growth

• Placenta is a temporary organ that forms during pregnancy to provide oxygen, nutrients, and waste removal for the fetus via the umbilical cord
• Placenta also produces hormones (hCG, estrogen, progesterone) that maintain the pregnancy and prepare the mother's body for birth
• Fetus is the term used for the developing human from the 9th week of pregnancy until birth, characterized by further growth and maturation of organs and systems

Gestation and Pregnancy Stages

• Gestation is the period of fetal development from conception to birth, typically lasting about 40 weeks in humans
• First trimester (weeks 1-12) involves rapid cell division, implantation, and early organ development
• Second trimester (weeks 13-27) is characterized by fetal growth, movement, and continued organ maturation
• Third trimester (weeks 28-40) involves final fetal growth and preparation for birth

Birth and Postpartum

Labor and Delivery

• Labor is the process of giving birth, characterized by regular uterine contractions that dilate the cervix and push the fetus through the birth canal
• Labor has three stages: dilation of the cervix, delivery of the baby, and delivery of the placenta
• Parturition (childbirth) occurs when the fetus is expelled from the uterus through the vagina, typically after about 40 weeks of gestation

Postpartum Changes and Lactation

• Postpartum period refers to the time after childbirth when the mother's body undergoes changes to return to its pre-pregnancy state
• Uterine involution is the process of the uterus shrinking back to its normal size, which takes about 6-8 weeks
• Lactation is the production and secretion of milk from the mammary glands to nourish the newborn
• Prolactin stimulates milk production, while oxytocin triggers milk ejection (let-down reflex) in response to infant suckling