Fetal Development

Fetal Development

Genetics

CHROMOSOMES

• Threadlike, spiral structures found in the nucleus of the cell
• Each cell in the human body contains 46 chromosomes
• Two types of cell division:

 mitosis – the exact duplication of the DNA content in the nucleus of the cell. Produces two identical cells
 meiosis - occurs in the sex cells; cells undergo 2 sequential divisions whereby the number of chromosomes is 1st reduced by half so that chromosomes are inherited from both the mother and father; and then continue with regular cell division or duplication

GENES
• In each gene, a segment of the DNA chain, is coded for inherited traits
• Genes carry instructions for dominant & recessive traits
• Only 1 dominant gene is needed to express the trait
• If only 1 parent has a dominant gene for a trait there is a 50% chance that each child will have the trait

SEX DETERMINATION
• Determined at fertilization
• Sperm contain both an x and a y chromosome
• Eggs only contain x chromosomes
• If the sperm donates an x chromosome the baby will be a girl = xx
• If the sperm donates a y chromosome the baby will be a boy = xy

Embryonic Development

FETAL DEVELOPMENT
• Implantation – usually occurs in the upper segment of the uterus
 after implantation teratogenic agents may be very damaging to the developing embryo
• Embryonic cell differentiation
• Fetal membranes and amniotic fluid
 2 membranes begin to form at the time of implantation: amnion which is the inner layer & the chorion is the outer layer
 later they fuse to form the amniotic sac or bag of waters
 normal volume of amniotic fluid is 800-1000ml. If > 2 liters is termed hyramnios & is associated with malformations of the fetal CNS & GI tracts


PLACENTA
• permits the exchange of materials between fetus & mother – fully formed is a flat disk about 1” thick & weighs 500 grams
o Functions:
 Placental transfer: movement of gases, nutrients, waste materials, drugs & other substances across the placenta from maternal to fetal circulation & back
 Immunologic functions: protection of the fetus from some pathogens & prevention of rejection by the mother
 Blood flow: can be reduced if uterine artery is constricted due to HTN, during strong contractions or when mother is in supine position
 Endocrine functions: production of hCG for maintenance of pregnancy; hPL which produces changes in the mother’s metabolic processes; progesterone to maintain the thick lining of the uterus & development of the breasts for lactation.

Embryonic Development

FETAL CIRCULATION –
• Fetus receives oxygen from the mother via the placenta so lungs do not perform this function in utero
• Ductus arteriosus – connects the pulmonary artery with the aorta, bypassing the lungs
• Ductus venosus – bypasses the liver
• Foramen ovale – located between the two atria of the heart; bypasses the right ventricle which would send blood to the lungs

CIRCULATION AFTER BIRTH

• Clamping of the umbilical cord
• Infant takes 1st breath
• Blood goes to the lungs
• Foramen ovale closes
• Ductus arteriosus closes
• Ductus venosus closes

FETAL DEVELOPMENT
• Organogenesis period
• Embryo – 1st eight weeks
 most critical time period while major organ systems are forming; may be affected by drugs or infections
• Fetus – after 8 weeks to birth
• Postterm or postmature infants – if born past 42 weeks
 have long nails, lots of hair on head, less vernix & fat, very dry skin
 major problems may occur due to the “old” placenta

Multifetal Pregnancy