Prenatal Development
Multiple Births
Monozygotic (identical) twins: genetically identical; formed when a cluster of cells splits apart within the first two weeks of development. Monozygotic → one zygote splits into two embryos
Dizygotic (fraternal) twins: no more genetically similar than any two siblings; occur when two ova are fertilized by two separate sperm. Dizygotic → two zygotes form separately
3+ births (triplets, quadruplets, etc.): can be produced either monozygotically, dizygotically, or both.
Twin Studies and Adoption Studies
Twin studies compare average similarity of monozygotic twins on a given outcome to that of dizygotic twins on a different outcome to estimate genetic influence.
Adoption studies compare an individual to their twin who has a different developmental environment to parse genetic vs. environmental effects.
Genotype vs Phenotype
Genotype: the combination of genetic material present in an organism.
Phenotype: the observable traits of the organism.
Prenatal Testing and Counseling
Ultrasound sonography: uses high-frequency sound waves to scan the mother’s womb and produce an image of the fetus.
Genetic Counseling: tests for inherited disorders by assembling a karyotype, which is a chart with photos of the chromosomes.
Chorionic Villus Sampling (CVS): performed at 8-10 weeks; a needle is inserted into the placenta to collect fetal cells for testing.
Amniocentesis: performed between the 15-20 weeks of pregnancy; a procedure that examines the fetal cells in the amniotic fluid.
Nature vs Nurture and Epigenetics
The ongoing Nature vs Nurture debate explores how genes and environment interact in development.
Epigenetics: changes in gene expression that do not involve changes to the underlying DNA sequence.
Histone modification: chemical alterations to histones (the proteins around which DNA is wrapped).
DNA methylation: addition of a methyl group to cytosine bases in DNA, which can suppress gene expression.
Epigenetics and Environment
Epigenetic factors help explain how environment can influence gene expression and ultimately phenotype without changing the DNA sequence itself.
Multifactorial Transmission
A given genotype can express a range of phenotypes depending on infinite combinations of environmental factors.
Gene-Environment Interaction: Scarr’s Theory
Your genes do not live in a vacuum; your genes influence your environment, and the environment can influence gene expression in turn. Sandra Scarr outlined two main mechanisms:
Children gravitate toward environments that align with their genetically influenced abilities (gene–environment correlation).
Temperament and genetic predispositions in a child evoke particular environmental responses (evocative gene–environment correlation).
Preconception Epigenetics and Health
Epigenetic effects of stress can occur before conception.
Preconception health of both mothers and fathers matters for offspring outcomes. A narrative review (Nutrients, 2024) highlights:
Maternal and paternal preconception nutrition and body weight influence offspring health; maternal nutrition is pivotal for fetal development, but paternal health also impacts offspring via epigenetic changes in sperm (DNA methylation and non-coding RNA).
Nutrient imbalances in preconception in both parents can lead to low birth weight and higher risk of metabolic diseases in children.
Deficiencies in folic acid, iron, iodine, and vitamin D are linked to developmental disorders.
Maternal obesity increases risk of chronic diseases in children.
Future research should prioritize human studies and integrated early interventions that address both maternal and paternal factors to reduce metabolic syndrome risks in future generations.
Key takeaway: parental health before conception can shape offspring health through epigenetic mechanisms; interventions should involve both parents.
Early Development: Key Stages and Terms
Gametes: reproductive cells containing genetic information.
Zygote: formed when sperm fuses with ovum; one hour after fertilization, the two gametes have fused to create a single cell.
Embryo: rapidly developing group of cells; from the end of the second week through the eighth week.
Fetus: rapid body growth and maturation of organs; from the ninth week onward.
The Germinal Stage (Fertilization to 2 Weeks)
Fertilization: joining of sperm and ovum to create a zygote.
Zygote travels to the uterus and implants into the uterine wall.
A protective layer forms around the zygote; the placenta is created, providing nourishment and oxygen through the umbilical cord.
The Embryonic Stage (2 Weeks to 8 Weeks)
The zygote becomes an embryo and differentiates into three germ layers:
Ectoderm: forms skin, hair, teeth, sense organs, brain, and spinal cord.
Endoderm: forms digestive system, liver, pancreas, and respiratory system.
Mesoderm: between the two layers; forms muscles, bones, and blood.
The Fetal Stage (8 Weeks to Birth)
Dramatic increases in size and weight; organs become operational.
By around 3 months, the fetus swallows and urinates.
Hormones released during development differentiate male and female.
Mitigating Threats to the Fetus
Varied, healthy maternal diets are associated with fewer pregnancy complications, easier labor, and healthier babies.
Maternal mental health and maternal support are important protective factors.
Threats to the Fetus: Teratogens
Teratogens: environmental factors that can cause developmental abnormalities.
Why research on teratogens is challenging:
Ethical concerns in studying pregnant people.
Timing and sensitivity of exposure (critical and sensitive periods).
Individual variability and complex interactions among factors.
Alcohol and Fetal Development
Fetal Alcohol Spectrum Disorder (FASD): about 1750 infants affected.
Alcohol use during pregnancy is associated with delayed growth, intellectual disability, and deformities.
Global context: public health messaging emphasizes avoiding alcohol during pregnancy; research supports ongoing monitoring and counseling.
Cannabis (Weed) During Pregnancy
Evidence on marijuana as a teratogen is more limited than for alcohol.
Some studies suggest THC exposure may be linked to low birth weight and neurological issues.
Cannabis should be avoided during breastfeeding; the risks of CBD alone remain largely undetermined.
Cocaine, Opiates, and Prescription Opioids
Cocaine: risks include premature birth, placental abruption, neurological issues, seizures; can be transmitted through breast milk.
Opiates: risks include placental abruption, premature birth, preeclampsia, neonatal opiate withdrawal syndrome.
In 2019, about 7\% of women reported use of prescription opioid pain relievers during pregnancy.
Neonatal Withdrawal Syndrome (NAS)
NAS: cluster of symptoms that can occur as newborns withdraw from substances (e.g., opioids) exposure before birth.
Symptoms: seizures, poor feeding, sleep disruption, dehydration and vomiting, hyperactive reflexes.
Environmental Toxins
External toxins: radon, mercury, phthalates, pesticides.
Bisphenol A (BPA): known endocrine disruptor used in plastics since the 1950s; exposure is ubiquitous in environments and the human body.
Very-low-dose BPA exposure has been linked to adverse health effects (breast and prostate cancer, obesity, neurobehavioral problems, reproductive abnormalities).
Policy implications involve reforming chemical safety policies and considering broader public health protections.
Mercury and Fish Consumption
Mercury levels in fish can vary; excessive consumption during pregnancy may risk fetal brain development.
Guidance from health organizations emphasizes choosing low-mercury fish and limiting high-mercury options (e.g., some tuna variants).
Stress and the HPA Axis
Stress during pregnancy can affect fetal development via elevated cortisol, which can disrupt the HPA axis.
Chronic stress or extreme events (natural disasters, ongoing life stress) are particularly impactful.
The Mozart Effect: Myth or Reality?
The idea that listening to Mozart prenatally can increase intelligence has limited support.
What is supported:
Fetuses can perceive rhythm and respond to sounds.
Music can have positive effects, but there is nothing uniquely special about Mozart or classical music for IQ gains.
Fetal Perception and Preferences
Fetuses can perceive stimuli and may develop food preferences before birth.
Yes, fetuses can build food preferences.
Practical and Ethical Considerations for Practice
Clear, consistent public health messaging about alcohol and other teratogens is essential.
Routine screening for substance use in pregnancy should be considered as part of prenatal care.
Healthcare providers should be educated on current evidence about teratogens and preconception health, including the roles of both maternal and paternal factors in offspring health.
Summary of Key Takeaways
Prenatal development progresses through germinal, embryonic, and fetal stages with distinct milestones and vulnerabilities.
Genetics and environment interact in complex ways (multifactorial transmission; gene–environment correlations and epigenetics).
Prenatal testing (ultrasound, CVS, amniocentesis) provides information about inherited conditions and fetal health.
Maternal and paternal health before conception, including nutrition and weight, can epigenetically influence offspring health.
Teratogens (alcohol, tobacco, drugs, environmental toxins) pose risks; timing and individual variability influence impact.
Public health approaches should focus on education, prevention, and support for families to optimize prenatal outcomes.
Fetuses can perceive sensory inputs and may develop preferences; myths such as the Mozart effect have limited support, though exposure to rhythm and sound is meaningful.