Prenatal Development: Stages, Timelines, and Teratogens (sheilds 12pm)

Prenatal development = growth from conception (fertilization) to birth; foundation for physical and cognitive development; can be affected by prenatal exposures with lifelong impacts.

Trimester = pregnancy divided into three relatively equal segments; each with different fetal experiences and maternal changes.
First trimester: early signs (nausea/morning sickness), fatigue; greatest risk of miscarriage.
Second trimester (the golden period): reduced morning sickness, more energy; fetal movements felt (quickening); maternal–fetal bonding; anatomy scans (3D/4D ultrasounds).
Third trimester: nearing birth; increased discomfort and limited mobility; variability in length; possible induction or C-section; growth preparation for birth.

Fertilization occurs in the fallopian tube; formation of a zygote (single cell) with a unique genetic code from both parents.
Sex determined at conception by the 23rd chromosome pair: XX = female, XY = male.
Zygote undergoes rapid cell division and becomes a blastocyst; inner cell mass forms the embryo, outer layer forms the placenta.
Implantation into the uterine wall is crucial; failure leads to early pregnancy loss.
Ectopic pregnancy = fertilized egg implants outside the uterus (often in a fallopian tube); dangerous if not treated.

Major organ systems begin to develop (organogenesis): brain, heart, limbs, facial features.
Neural tube forms and becomes brain/spinal cord; heartbeat appears around $6$ weeks.
Embryo is highly vulnerable to teratogens; timing matters (different organs have different critical periods).

Rapid growth and refinement of organs and body structures.
Brain development accelerates: synaptogenesis (formation of new neural connections) and myelination (insulation of nerve fibers).
Fetus becomes responsive to stimuli: hearing sounds, touch, light to a limited extent; external movements become noticeable to others.
Viability threshold around $22-23$ weeks: ability to survive outside the womb with medical support; not guaranteed functional independence.
Third trimester involves fat deposition and weight gain preparing for birth.

Hormonal changes support gestation; maternal nutrition and behaviors influence fetal development.
Key risk factors include: folic acid deficiency, extreme maternal age, and various exposures.
Stress effects are not fully understood; avoiding excessive shame and seeking support is important.
Genetic factors and familial history can indicate risk for chromosomal abnormalities (e.g., Down syndrome, Turner syndrome) or single-gene mutations; genetic testing and counseling may be offered.
Common teratogens include alcohol, nicotine, prescription drugs, illicit drugs, and infections; effects depend on dose, timing, and fetal susceptibility.

Alcohol
- Fetal alcohol spectrum disorders; facial anomalies, growth retardation, intellectual disability; no safe amount during pregnancy.
Nicotine (cigarettes, vaping)
- Reduces fetal oxygen; associated with low birth weight, prematurity, respiratory problems.
Prescription and illicit drugs
- Methotrexate (cancer treatment) can be harmful during pregnancy; some antidepressants/anti-anxiety meds require medical review.
- Illicit drugs (heroin, cocaine, methamphetamine) can cause neonatal withdrawal and long-term effects.
Infections and malnutrition
- Infections and nutrient deficiencies (e.g., folic acid) can disrupt development; timing matters for specific organs.

Zygote: single fertilized cell containing combined genetic material.
Blastocyst: early multicellular stage with inner cell mass (becomes embryo) and outer layer (becomes placenta).
Implantation: blastocyst embeds in the uterine wall; essential for continued pregnancy.
Placenta: nourishes developing baby via maternal blood supply.
Quickening: subjective fetal movements felt by the mother.
Viability: the point when the fetus can potentially survive outside the womb (~ $22-23$ weeks) with intensive care.
Teratogen: any agent that can disturb development and cause birth defects, depending on timing and dose.

Know the three prenatal stages and their timing: germinal (weeks $1-2$ ), embryonic (weeks $3-8$ ), fetal (weeks $9$ to birth).
Understand what happens in each stage (implantation, organogenesis, brain development).
Recognize how trimester experiences differ and why second trimester is called the golden period.
Identify major teratogens and why timing and dose matter for outcomes.
Remember viability concept and why very early births require extensive medical support.

Open communication with healthcare providers about medications, alcohol, tobacco, and other exposures is essential for a safer pregnancy.
Genetic counseling/testing may be offered, especially with family history of disorders or when using reproductive technologies such as IVF.