Pregnancy & Human Development: Study Notes

Pregnancy & Human Development

Introduction

• Pregnancy: The sequence of events from fertilization to birth.

• Conceptus: The developing offspring during pregnancy.

• Gestation: The period from the last menstrual period to birth (approximately 280 days).

Fertilization

Gamete Viability

• Oocyte viability: 12–24 hours after ovulation.

• Sperm viability: 24–48 hours after ejaculation.

• Optimal fertilization window: Intercourse ≤2 days before ovulation or ≤24 hours after ovulation.

Sperm Transport & Challenges

• Sperm must reach the oocyte in 1–2 hours, but most fail due to:

  • Leakage from the vagina

  • Destruction by acidic vaginal environment

  • Blocked passage through cervical mucus

  • Phagocytosis by uterine leukocytes

  • Incorrect direction within the uterus

Capacitation: Sperm Maturation in Female Reproductive Tract

• Occurs over 8–10 hours as sperm travel through cervical mucus, uterus, and uterine tubes.

• Involves:

  • Increased motility

  • Removal of membrane proteins, enabling acrosomal reaction (sperm penetration)

Penetration of the Oocyte

• Oocyte surrounded by:

  • Corona radiata (outermost, contains ZP3 receptors)

  • Zona pellucida (glycoprotein-rich layer)

• Steps:

  1. Sperm binds ZP3 receptors, triggering acrosomal reaction.

  2. Acrosomal enzymes (hyaluronidase, acrosin) digest zona pellucida.

  3. One sperm fuses with oocyte membrane; triggers cortical reaction (release of zonal inhibiting proteins to prevent polyspermy).

Completion of Meiosis II

• Oocyte completes meiosis II, forming the ovum and a polar body.

Formation of the Zygote

• Fertilization: Genetic material from sperm and egg unite.

• Zygote: First cell of the new individual.

Pre-Embryonic Development

1. Cleavage

• Rapid mitotic divisions; cells become smaller.

• Morula: Solid ball of cells (16+ cells) by day 3.

2. Blastocyst Formation

• Blastocyst: Hollow ball of cells with a fluid-filled cavity (blastocoel).

• Consists of:

  • Trophoblast: Forms placenta and extraembryonic membranes.

  • Inner cell mass: Forms embryo.

3. HCG Production

• Trophoblast secretes human chorionic gonadotropin (hCG) to maintain corpus luteum and progesterone secretion, preventing menstruation.

4. Implantation

• Blastocyst embeds in endometrium (about 7 days after ovulation).

5. Trophoblast Differentiation

• Forms chorion and chorionic villi, which become part of the placenta.

Placental Development & Function

• Temporary organ; fully functional by end of 3rd month.

• Site of nutrient and waste exchange between embryo and maternal blood (no mixing of blood).

Placental Hormones

• hCG: Maintains corpus luteum.

• Estrogens & Progesterone: Increase throughout pregnancy; maintain uterine lining, suppress FSH/LH, prepare mammary glands.

• Human Placental Lactogen (hPL): Promotes fetal growth, breast maturation, and glucose-sparing for fetus.

• Human Chorionic Thyrotropin: Increases maternal metabolism.

• Relaxin: Relaxes pelvic ligaments and pubic symphysis for childbirth.

Embryonic Development

Gastrulation

• Inner cell mass forms upper epiblast and lower hypoblast, creating the embryonic disc.

• Formation of three primary germ layers:

  • Ectoderm: Nervous system, skin

  • Mesoderm: Muscles, bones, circulatory system

  • Endoderm: Digestive, respiratory, and urinary tracts

Extraembryonic Membranes

1. Amnion

• Transparent sac filled with amniotic fluid; cushions and protects embryo.

• Amniotic fluid volume at term: ~800 mL.

2. Yolk Sac

• Forms part of gut, produces early blood cells and vessels.

3. Allantois

• Forms base of umbilical cord; involved in early blood formation.

4. Chorion

• Forms part of placenta; contributes to fetal portion of placenta.

Fetal Period (Weeks 9–38)

• Organ systems grow and mature.

• Rapid body growth occurs.

Initiation of Labor (Parturition)

Hormonal Triggers

• Fetal cortisol and increased estrogen levels in mother stimulate labor.

• Estrogen increases oxytocin receptor formation in uterus.

• Fetal lungs secrete surfactant, softening cervix.

Prostaglandins & Oxytocin

• Fetal oxytocin stimulates placenta to release prostaglandins, enhancing uterine contractions.

Positive Feedback Loop

• Maternal uterine contractions stimulate hypothalamus to release more oxytocin, increasing contractions further.

Stages of Labor

1. Dilation Stage

   • Onset of true contractions to full cervical dilation (~10 cm).

   • Longest stage (6–12 hours or more).

   • Amniotic sac ruptures (“water breaks”).

2. Expulsion Stage

   • From full dilation to delivery of infant (20–50 minutes for first birth).

   • Infant exits head first (vertex position).

3. Placental Stage

   • Delivery of placenta (~30 minutes after birth).

   • All placental fragments must be removed to prevent hemorrhage.

Neonatal Period & Apgar Score

• Neonatal period: First 4 weeks of life.

Apgar Test

• Done at 1 and 5 minutes after birth (sometimes at 10 minutes).

• Assesses:

  • Heart rate

  • Respiration

  • Muscle tone

  • Reflexes

  • Color

• Each scored 0–2; total score out of 10.

Maternal Physiologic Changes During Pregnancy

Anatomical Changes

• Increased blood flow to reproductive organs.

• Uterus expands, displacing abdominal organs.

• Lordosis (increased lumbar curvature) to balance abdominal weight.

Common Symptoms

• Nausea/morning sickness (due to increased estrogen and progesterone).

• Heartburn (from displacement of stomach).

• Increased urination (due to increased metabolism and fetal waste removal).