29.1 Overview of the Prenatal Period

Stages of Prenatal Development

Prenatal development is divided into three main periods, each characterized by specific developmental milestones and changes in the embryo or fetus.

• Pre-embryonic Period (First 2 Weeks):

  • Begins with fertilization.

  • The zygote undergoes cleavage, forming a blastocyst.

  • Ends with implantation in the uterine lining.

• Embryonic Period (Weeks 3-8):

  • Rudimentary versions of organs appear.

  • Organogenesis begins; the structure is now called an embryo.

• Fetal Period (Weeks 9-38):

  • The organism is now called a fetus.

  • Rapid growth and tissue maturation occur.

29.2 Fertilization and Early Embryonic Development

Fertilization

Fertilization is the process by which a sperm cell fuses with a secondary oocyte to form a zygote, restoring the diploid number of chromosomes and determining the sex of the organism.

• Fusion of sperm and secondary oocyte.

• Restores diploid number of chromosomes (46 total).

• Determines the sex of the organism.

Phases of Fertilization

1. Corona Radiata Penetration: Sperm push through the outer layer.

2. Zona Pellucida Penetration: Acrosome reaction allows one sperm to penetrate.

3. Fusion of Sperm and Oocyte: Pronuclei fuse, forming a zygote.

Cleavage & Blastocyst Formation

• Cleavage: Rapid mitotic divisions increase cell number without increasing overall size.

• Morula (16-cell stage): Pre-embryo continues division.

• Blastocyst: Fluid-filled cavity forms; trophoblast (outer cells) develops into placenta, and embryoblast (inner cells) forms the embryo.

Implantation

• Blastocyst embeds in uterine wall (day 7).

• Human Chorionic Gonadotropin (hCG): Secreted to maintain the corpus luteum for hormone production.

• Placenta begins forming by the 2nd week.

29.3 Embryonic Development

Gastrulation (Week 3)

Gastrulation is the process by which the three primary germ layers form, setting the stage for organ development.

• Ectoderm: Becomes nervous system and skin.

• Mesoderm: Becomes muscle, bone, and connective tissue.

• Endoderm: Becomes digestive, respiratory, and urinary linings.

Embryonic Folding (Weeks 3-4)

• Cephalocaudal Folding: Head and tail regions fold inward.

• Transverse Folding: Left and right sides fold inward, creating a cylindrical body.

Placenta Development

• Forms from maternal endometrium and fetal chorion.

• Functions:

  • Nutrient, gas, and waste exchange.

  • Secretes hormones (hCG, estrogen, progesterone).

  • Transmits maternal antibodies.

29.4 Fetal Period (Weeks 9-38)

Growth and Organ Maturation

• Crown-rump length (CRL) and weight increase significantly.

• Movement begins (quickening).

• Last two months: Rapid weight gain, lung maturation.

29.5 Maternal Adaptations to Pregnancy

Hormonal Changes

• hPL (Human Placental Lactogen): Alters metabolism for fetal glucose supply.

• Oxytocin: Prepares for labor and milk ejection.

Body System Changes

• Cardiovascular: Blood volume increases 50%.

• Respiratory: Tidal volume increases.

• Urinary: Increased filtration due to fetal waste.

• Digestive: Increased nutrient absorption, but also reflux, nausea.

29.6 Labor and Childbirth

Stages of Labor

1. Dilation Stage:

   • Longest stage; cervix dilates to 10 cm.

   • Amniotic sac may rupture ("water breaking").

2. Expulsion Stage:

   • Baby is pushed out of the birth canal.

3. Placental Stage:

   • Placenta is expelled (afterbirth).

True vs. False Labor

• False Labor (Braxton-Hicks): Irregular, non-progressive contractions.

29.7 Postnatal Changes in the Newborn

Neonatal Adjustments

• Respiratory System: First breath expands lungs.

• Circulatory System: Fetal shunts close.

• Thermoregulation: Brown fat helps generate heat.

• Digestive System: Meconium (first stool) is passed.

29.8 Maternal Postpartum Adjustments

Hormonal Changes

• Estrogen & Progesterone Drop: Mood swings, postpartum depression risk.

• Prolactin & Oxytocin: Promote breastfeeding.

Lactation

• Colostrum (initial milk): Rich in antibodies.

• Milk Letdown: Oxytocin stimulates release via positive feedback.

29.9 Human Genetics and Inheritance

Chromosomes & Heredity

• Humans have 46 chromosomes (23 pairs).

• Genotype: Genetic makeup; Phenotype: Physical traits.

Patterns of Inheritance

• Dominant-Recessive: Dominant allele expressed (e.g., eye color).

• Incomplete Dominance: Blended traits (e.g., sickle cell trait).

• Codominance: Both alleles expressed (e.g., ABO blood type).

• Polygenic Inheritance: Multiple genes influence traits (e.g., height, skin color).

• X-linked Inheritance: Traits carried on the X chromosome (e.g., color blindness, hemophilia).