10.4 Pregnancy and fetal development

Pregnancy and fetal development are crucial processes in human reproduction. From fertilization to birth, the journey involves complex biological changes in both mother and developing baby. Understanding these stages helps us appreciate the intricacies of human life formation.

This section explores key milestones in pregnancy, including fertilization, implantation, and fetal growth. We'll also look at the placenta's vital role and potential complications that can arise during this remarkable nine-month journey.

Fertilization and Implantation

Zygote Formation and Early Development

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• Sperm cell penetrates and fuses with an ovum in the ampulla of the uterine tube, creating a diploid zygote
• Zygote undergoes rapid mitotic cell divisions (cleavage) as it moves through the uterine tube toward the uterus
  • Forms a solid ball of cells called a morula
• Morula develops into a blastocyst, a hollow ball of cells
  • Inner cell mass (embryoblast) will differentiate into the embryo
  • Outer layer of trophoblast cells will form the extraembryonic membranes and placenta

Implantation Process

• Blastocyst adheres to and invades the endometrium of the uterus 6-7 days after fertilization
• Trophoblast cells secrete enzymes that allow the blastocyst to burrow into the endometrium
  • Blastocyst becomes completely covered by the endometrium
  • Trophoblast cells differentiate and proliferate, forming the placenta and chorionic villi
• Inner cell mass differentiates into the embryo, which will develop into the fetus

Placenta Development and Function

Placental Structure and Formation

• Temporary organ formed from trophoblast cells of the blastocyst and maternal endometrium
• Chorionic villi, finger-like projections of the trophoblast, invade the endometrium
  • Contain fetal blood vessels
  • Maternal blood pools in the spaces between the chorionic villi
• Placenta connected to the fetus by the umbilical cord
  • Contains two umbilical arteries and one umbilical vein

Placental Functions

• Allows exchange of oxygen, nutrients, hormones, and antibodies from mother to fetus
  • Waste products (carbon dioxide) removed from fetus to mother
  • Substances diffuse between maternal and fetal blood without direct mixing
• Placental membrane serves as a selective barrier
  • Allows some substances to pass through while blocking others (teratogens, certain medications)
  • Provides protection for the developing fetus
• Endocrine gland function: secretes hormones to maintain pregnancy and support fetal development
  • Human chorionic gonadotropin (hCG), progesterone, and estrogen

Fetal Development Milestones

First Trimester (Weeks 1-12)

• Embryonic period (weeks 1-8): rapid development of major organ systems
  • Neural tube, heart, and limb buds form
  • Embryo is most susceptible to teratogens during this period
• Fetal period begins at week 9: continued growth and development of organs and tissues
  • Fetal movements can be detected by the end of the first trimester

Second Trimester (Weeks 13-28)

• Rapid fetal growth and development of features (hair, nails, and skin)
• Fetal hearing develops, and the fetus can respond to sounds
• Fetal movements become more pronounced and can be felt by the mother (quickening)
• Vernix caseosa, a protective waxy coating, covers the fetal skin
• Fetal lungs begin to develop alveoli and produce surfactant

Third Trimester (Weeks 29-40)

• Continued fetal growth and accumulation of fat deposits
• Fetal brain undergoes rapid development (formation of gyri and sulci)
• Fetal lungs mature and prepare for breathing after birth
• Fetus typically assumes a head-down position in preparation for birth
• Fetal bones harden, except for the skull bones, which remain flexible for passage through the birth canal

Pregnancy Complications and Impacts

Maternal Health Complications

• Gestational diabetes: high blood sugar levels during pregnancy
  • Can lead to macrosomia (large baby), increased risk of cesarean delivery, and neonatal hypoglycemia
• Preeclampsia: high blood pressure and protein in the urine after 20 weeks of pregnancy
  • Can cause maternal organ damage and fetal growth restriction if left untreated
• Ectopic pregnancy: implantation of the embryo outside the uterus (usually in the uterine tube)
  • Can cause life-threatening maternal hemorrhage if not detected and treated early
• Placental abruption: premature separation of the placenta from the uterine wall
  • Can cause maternal hemorrhage and fetal distress or death
• Placenta previa: placenta partially or completely covers the cervix
  • Can cause severe maternal bleeding and requires cesarean delivery

Fetal Health Complications

• Preterm labor: onset of labor before 37 weeks of gestation
  • Can lead to premature birth and associated complications (respiratory distress syndrome, intraventricular hemorrhage, necrotizing enterocolitis)
• Miscarriage: spontaneous loss of pregnancy before 20 weeks of gestation
  • Most often due to chromosomal abnormalities in the embryo or fetus
• Intrauterine growth restriction (IUGR): fetus fails to reach expected growth milestones
  • Can be caused by maternal factors (hypertension, smoking) or placental insufficiency
• Congenital anomalies: structural or functional defects present at birth
  • Can be caused by genetic factors, teratogens, or a combination of both