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Fetal Development
Two Major Categories of Cells
• Soma cells
– Make up the organs and tissues of the human body
• Gametes (germ cells or sex cells)
– Found in the reproductive glands only
Types of Cellular Division
• Mitosis
– The process by which somatic (body) cells give birth
to daughter cells
– Each daughter cell contains the same number of
chromosomes as the parent cell
• Meiosis
– The process by which gametes undergo two
sequential cellular divisions of the nucleus
(gametogenesis)
Gender Determination
• A female fetus (XX) will develop when the ovum unites
with a spermatozoon with an X chromosome
• A male fetus (XY) will develop when the ovum unites with
a spermatozoon with a Y chromosome
– There is approximately a 50/50 chance of either
occurrence
Three Stages of Human Development
During Pregnancy
• Pre-embryonic
– Begins at fertilization and lasts through the end of
the second week after fertilization
• Embryonic
– Begins approximately two weeks after fertilization
and ends at the conclusion of the 8th week after
fertilization
• Fetal
– Begins at 9 weeks after fertilization and ends at birth
Processes of the Pre-Embryonic Stage of
Development
• Fertilization
– Occurs when the sperm penetrates the ovum
• Cellular reproduction
– The zygote begins the process of mitotic division
known as cleavage
• Implantation
– The trophoblast develops fingerlike projections that
help the blastocyst to burrow itself into the nutrientrich endometrium
Three Germ Layers of the Embryo
• Ectoderm: Outer layer of cells
– Forms skin, hair, nails, and the nervous system
• Mesoderm: Middle layer
– Forms the skeletal, muscular, and circulatory
systems
• Endoderm: Inner layer
– Forms the glands, lungs, and urinary and digestive
tracts
Development of Supportive Structures
• Fetal membranes
• Amniotic fluid
• Placenta
• Umbilical cord
Four Main Functions of Amniotic Fluid
• Physical protection
• Temperature regulation
• Provision of unrestricted movement
• Symmetrical growth
Functions of the Placenta
• Provides for the transfer and exchange of substances
• Acts as a barrier to certain substances
• Functions as an endocrine gland by producing hormones
Fetal and Placental Circulation
• Fetal circulation
– Low oxygen tension
– High hematocrit
– Three fetal shunts
• Ductus venosus
• Foramen ovale
• Ductus arteriosus
• Placental circulation
– Exchange occurs in the intervillous spaces
Special Considerations of Fetal
Development
• Effects of teratogens on the developing fetus
• Ectopic pregnancy
• Multi-fetal pregnancy
Types of Teratogens
• Ingested
– Prescription and over-the-counter medications, illicit
drugs, and alcohol
• Infectious
– Varicella, cytomegalovirus (CMV), and rubella
• Environmental substance
– Ionizing x-rays, radioactive substances, and certain
chemicals
Ectopic Pregnancy
• Definition
– The zygote implants in places other than the uterus
– 95% occur in the fallopian tube
• Causes
– Blockage or scarring of the fallopian tubes either
from infection or trauma
• Occurrence
– 19.7 of 1000 pregnancies
Categories of Twins
• Monozygotic Twins
– Identical twins derived from one zygote
– Share same genetic material; always the same sex
• Dizygotic twins
– Fraternal twins develop from separate egg and sperm
fertilizations
– Genetic material is not identical; may or may not be
the same sex
Classification of Twins
• Diamniotic-dichorionic twins
– Each develop in their own amniotic sac
– Placentas do not share any vessels
• Diamniotic-monochorionic twins
– Each have their own amniotic sac but share a
common chorionic sac
– Each have a separate placenta but the placentas
share some vessels
• Monoamniotic-monochorionic
– Have one amniotic cavity that they both share
– High risk - cord entangelment, twin-twin transdusion
Factors Increasing Chance of Having
Twins
• Family history of twins
• Recent stoppage of oral contraceptive
• Tall or large stature of the mother
• African-American heritage
• Use of fertility medications