Prenatal Development

Prenatal Development

Overview

  • Prenatal development refers to the stages from conception to birth.

  • It takes approximately 266 days for a single-celled zygote to develop into a fetus ready for birth.

  • Prenatal development is divided into three main phases:

    1. Period of the Zygote

    2. Period of the Embryo

    3. Period of the Fetus

Stages of Prenatal Development

1. Period of the Zygote

  • Duration: Conception to implantation (approximately 14 days).

  • Process:

    • A fertilized ovum (zygote) undergoes division by mitosis.

    • Cell differentiation begins.

    • Inner cells form the blastocyst, which becomes the embryo.

    • Outer cells, known as trophoblasts, become support structures for the embryo.

Implantation

  • Occurs 10-14 days after conception.

  • The blastocyst reaches the uterus, attaches to the uterine wall, and taps into the mother's blood supply.

  • Some zygotes may not survive this initial phase of development due to:

    • Failure to attach to uterine wall or attaching in a poor position.

    • Genetic abnormalities leading to developmental failure.

Cell Division and Development Timeline

  1. Single-celled mature ovum - discharged by ovary on days 9 to 16 of the menstrual cycle.

  2. Fertilization - occurs usually within 24 hours in the fallopian tube.

  3. 2 cells - (48 hours post-fertilization).

  4. 4 cells - (72 hours post-fertilization).

  5. 16 to 32 cells - (4 to 5 days post-fertilization).

  6. Implantation - (8 to 14 days post-fertilization).

2. Period of the Embryo

  • Duration: 3rd to 8th week after conception.

    • Key Developments:

    • Cell differentiation continues.

    • Germ Layers Formation:

      • Ectoderm: Develops into nervous system, skin parts, and sensory receptors.

      • Mesoderm: Forms muscles, bones, circulatory, excretory, and reproductive systems.

      • Endoderm: Develops into the digestive and respiratory systems.

    • The heart begins to beat by the end of the 4th week.

Support Structures

  • Amnion: A sac filled with amniotic fluid that cushions and regulates temperature.

  • Chorion: Surrounds the amnion and becomes the lining of the placenta.

  • Placenta: Provides oxygen and nutrients to the developing embryo.

  • Umbilical Cord: Connects the embryo to the placenta and uterine wall.

Neural Tube Development

  • At 3 weeks, the ectoderm folds into a structure called the neural tube, which eventually becomes the brain and spinal cord.

  • Neural Tube Defects:

    • Spina Bifida: Occurs if the neural tube fails to seal at the bottom.

    • Anencephaly: Occurs if the neural tube fails to seal at the top.

Month 2 Developments

  • The embryo takes on a more human-like appearance with developments such as:

    • Formation of eyes with corneas and lenses.

    • Well-formed ears.

    • Development of a rudimentary skeleton and limbs extending outward.

    • Brain growth leads to the first muscular contractions.

    • Initiation of sexual development and completion of organogenesis.

Conclusion of Period

  • By the end of this stage, all the structures present at birth have formed, at least in preliminary stages.

3. Period of the Fetus

  • Duration: 9th week until birth.

  • Key Features:

    • Functionality of major organ systems.

    • Rapid growth and refinement, with increasing coordination between systems.

    • The sex of the fetus can be determined via ultrasound by the end of the 3rd month.

Second Trimester

  • Duration: 4th, 5th, and 6th months of pregnancy.

  • Development of visual and auditory senses begins;

    • At this stage, there is little chance of survival outside the womb.

Third Trimester

  • Known as the "finishing phase."

  • Marks the age of viability, which is approximately 24 weeks, when some infants may survive if born prematurely.

Age of Viability

  • 24 weeks: Just reaches the threshold of viability.

  • 36 weeks: The fetus develops vernix (a protective coating) and lanugo (fine hair).

Summary of Periods

  • Period of the Zygote: Conception to implantation.

  • Period of the Embryo: 3rd week to the end of the 8th week, where major organs and anatomical structures are formed.

  • Period of the Fetus: From the 9th week to birth, marked by rapid growth, refinement, and integration of systems.

  • Age of viability occurs around 22-28 weeks.

Teratogens

  • Definition: Teratogens are any agents that may cause developmental deviations.

  • Types of teratogens include:

    • Environmental toxins.

    • Maternal characteristics.

    • Diseases and disorders affecting the mother.

    • Legal and illegal drugs.

Principles of Teratogens

  • The impact of a teratogen is greatest when an organ is forming (critical period).

  • The same outcome can be caused by different teratogens.

  • Different teratogens can result in a variety of defects.

  • The longer the duration of exposure or the higher the “dose,” the greater the effect.

  • Not all embryos or fetuses are equally affected by teratogenic exposure.

Specific Environmental Toxins

  • Lead: Associated with miscarriage and cognitive impairments.

  • Mercury: Linked to cerebral palsy.

  • Radiation: Can lead to miscarriage, stillbirth, microencephaly, and stunted growth.

  • Pesticides, food additives, Zincs, and PCBs: Other substances implicated in developmental risks.

Maternal Characteristics

  • Age: Increases the risk of genetic abnormalities.

  • Stress: Can cause oxygen deprivation and lead to premature and difficult births.

  • Nutrition: Inadequate maternal nutrition correlates with low birth weight and increased infant mortality.

Nutritional Needs

  • Synapse formation and myelination require significant metabolic energy during pregnancy.

  • A recommended increase of 300 calories per day during pregnancy, especially critical after the first 3-4 months.

Consequences of Poor Nutrition

  • Associated with lower IQ test scores, poorer school performance, slower language development, poor fine motor skills, and behavioral problems.

Maternal Diseases

  • Rubella: Can lead to deafness, blindness, cognitive impairments, cardiac disorders, and death.

  • Syphilis: Associated with blindness, cognitive impairments, and physical abnormalities.

  • AIDS: Results in neurological impairments, microcephaly, delays in physical and cognitive development, and death.

Drugs and Prenatal Development

  • Prescription and nonprescription drugs:

    • DES: Causes reproductive issues and cancer.

    • Aspirin: Linked to blood disorders in high doses.

    • Caffeine: Associated with preterm delivery and low birth weight in high doses.

    • Thalidomide: Leads to missing limbs and major physical abnormalities.

Alcohol

  • Impacts the fetus by causing oxygen deprivation; passes easily through the placenta, leading to increased rates of stillbirth and cognitive impairments in children.

  • Father's alcohol use can also lead to genetic damage resulting in birth defects.

Fetal Alcohol Syndrome

  • Symptoms include growth delays, facial abnormalities, central nervous system impairments, cognitive impairments, and behavioral problems, often associated with heavy, binge drinking during pregnancy.

Smoking/Nicotine

  • Increases the risk of miscarriage, stillbirth, premature births, and low birth weight. Also associated with risks for respiratory problems and cognitive/behavioral development issues.

  • The combination of smoking and drinking has been linked to Sudden Infant Death Syndrome (SIDS).

Other Drugs

  • Cocaine: Withdrawal symptoms, low birth weight, and attentional issues.

  • Heroin: Leads to withdrawal symptoms and low birth weight, impacting attention and behavior.

  • Marijuana: Associated with increased tremors and poor verbal/memory development.

Challenges in Studying Teratogens

  • Variability in susceptibility among individuals.

  • Critical and sensitive periods of fetal development.

  • Complex interactions between teratogens and outcomes.

  • Lack of uniform responses to teratogenic exposure.

  • Potential for sleeper effects—delayed outcomes.

Postnatal Assessments

The APGAR Scale

  • Assessment Indicators:

    • Respiratory Effort:
      0 - No breathing for more than one minute
      1 - Irregular and slow
      2 - Good breathing with normal crying

    • Body Color:
      0 - Blue and pale
      1 - Body pink, but extremities blue
      2 - Entire body pink

    • Heart Rate:
      0 - Absent
      1 - Slow (less than 100 beats per minute)
      2 - Fast (100-140 beats per minute)

    • Muscle Tone:
      0 - Limp and flaccid
      1 - Weak, inactive, with some flexion of extremities
      2 - Strong, active motion

    • Reflex Irritability:
      0 - No response
      1 - Grimace
      2 - Coughing, sneezing, and crying

The Brazelton Neonatal Behavioural Assessment Scale

  • Assessments include:

    1. Response decrement to repeated visual stimuli.

    2. Response decrement to rattle.

    3. Response decrement to bell.

    4. Response decrement to pinprick.

    5. Orienting responses to inanimate and animate visual and auditory stimuli.

    6. Quality and duration of alert periods.

    7. General muscle tone during rest and response to handling.

    8. Motor activity.

    9. Responses to cuddling and related behaviors.

    10. Assessment of irritability during examination and self-quieting attempts.