It was not until1758thatKaspar Wolffproposed thatboth parents contribute equally
to the structure of the baby.
I. S tages of Fetal Development
- In just 38 weeks, a fertilized egg (ovum) matures from a single cell carrying all
the necessary genetic material to a fully developed fetus ready to be born.
Fetal growth and development are typically divided into three periods:
● Pre-embryonic - first 2 weeks, beginning with fertilization
● Embryonic - week 3 to 8
● Fetal – week 8 through birth
Terms used to denote fetal growth:
Name Time Period
Ovum From ovulation to fertilization
Zygote From fertilization to implantation
Embryo From implantation to 5-8 weeks
Fetus From 5-8 weeks until term
FERTILIZATION: THE BEGINNING OF PREGNANCY
Fertilization
- is the union of an ovum and sperm, typically in the outer third of the fallopian
tube (ampullar portion).
Oneovum matures per month, and itis capable of fertilizationfor 24–48 hours.
Spermatozoacansurvive for about 48–72 hours.
ertilization window:sexual intercoursemustoccurwithin 72 hours (48 hours
F
before and 24 hours after ovulation).
Ovum
- is surrounded by the zona pellucida and corona radiata, which protect it and
aid its movement into the fallopian tube.
verage ejaculationcontains 50–200 million spermper milliliter(4
A 00 million per
ejaculation).
Spermreaches the cervix in 80 seconds and the fallopiantube within 5 minutes.
, permatozoamove through the cervix, uterus, and fallopian tubesusing flagella
S
and uterine contractions.
apacitationisa final process that sperm must undergoto be ready for
C
fertilization.This process, which happens as thesperm move toward the ovum,
consists of changes in the plasma membrane of the sperm head, which reveal
the sperm-binding receptor sites.
fter sperm penetration, theovum and sperm chromosomesfuse,forming a
A
zygote with 46 chromosomes.
If anX-carrying spermatozoonenters the ovum, theresulting child will have two X
chromosomes and will be female (XX). If aY-carryingspermatozoonfertilized the
ovum, theresulting child will have an X and a Y chromosomeand will be male (XY).
IMPLANTATION
fter fertilization, thezygote migrates to the uterusover 3–4 days, aided by
A
fallopian tube contractions.
itotic cell division (cleavage) begins, and by thetime it reaches the uterus, the
M
zygote consists of 16–50 cells and is called a morula.
hemorula continues to multiply,forming a blastocyst,with outer trophoblast cells
T
(future placenta) and inner embryoblast cells (future embryo).
Implantation occurs 8–10 days after fertilizationwhen the blastocyst attaches to
the uterine endometrium.
heprocess of implantationincludesapposition (contact),adhesion (attachment),
T
and invasion (settling into uterine folds).
Implantationusually happens high in the uterus;lowimplantation(p
lacenta
previa) can cause complications during birth.
Once implanted, thezygote is termed an embryo.
MBRYONIC AND FETAL STRUCTURES
E
The Decidua
- After fertilization, the corpus luteum continues to function due to human
chorionic gonadotropin (hCG) secreted by trophoblast cells.
- The uterine endometrium grows in thickness and vascularity instead of
sloughing off, and is now termed the decidua.
- The decidua has three parts:
, Decidua basalis (below the embryo),
●
● Decidua capsularis (encapsulates the trophoblast),
● Decidua vera (the remaining uterine lining).
- As the embryo grows, it pushes the decidua capsularis until it contacts the
decidua vera, eventually fusing.
- At birth, the entire inner uterine lining is stripped away, increasing the risk of
hemorrhage and infection.
Chorionic Villi
- Once implantation is achieved, the trophoblastic layer of cells of the blastocyst
begins to mature rapidly. As early as the 11th or 12th day, miniature villi, or
probing “fingers,” termed chorionic villi, reach out from the single layer of cells
into the uterine endometrium.
The Placenta
- The placenta (Latin for pancake, which is descriptiveof its size and
appearance at term), arises out of trophoblast tissue.
- Itserves as the fetal lungs, kidneys, and gastrointestinaltract and as a
separate endocrine organ throughout pregnancy.
Its growth parallels that of the fetus, growing from a few identifiable cells at
the beginning of pregnancy to an organ 15 to 20 cm in diameter and 2 to 3 cm
in depth at term.
- It covers about half the surface area of the internal uterus.
URPOSES OF THE PLACENTA
P
Circulation:
- By the12th day of pregnancy,maternal blood beginscollecting in the
intervillous spaces surrounding the chorionic villi.By the3rd week, oxygen
and nutrients like glucose, amino acids, and water pass from maternal blood
to the villi capillaries, which then transport them to the embryo.There is no
direct blood exchange between mother and embryo; instead,nutrients are
transferred via selective osmosis through the chorionic villi. Despite this
barrier,fetal cells and enzymes like alpha-fetoproteincan cross, as well as
most drugs,which is why pregnant women should avoidnonessential
drugs.
Endocrine Function:
- Aside fromserving as the source of oxygen and nutrientsfor the
fetus, thesyncytial (outer) layer of the chorionicvillidevelops into a separate,
important
hormone-producing system.
● Human Chorionic Gonadotropin:The hormone hCGis detectable
in maternal blood and urine shortly after implantation and is used
in pregnancy tests, thoughfalse negatives may occurbefore or after
to the structure of the baby.
I. S tages of Fetal Development
- In just 38 weeks, a fertilized egg (ovum) matures from a single cell carrying all
the necessary genetic material to a fully developed fetus ready to be born.
Fetal growth and development are typically divided into three periods:
● Pre-embryonic - first 2 weeks, beginning with fertilization
● Embryonic - week 3 to 8
● Fetal – week 8 through birth
Terms used to denote fetal growth:
Name Time Period
Ovum From ovulation to fertilization
Zygote From fertilization to implantation
Embryo From implantation to 5-8 weeks
Fetus From 5-8 weeks until term
FERTILIZATION: THE BEGINNING OF PREGNANCY
Fertilization
- is the union of an ovum and sperm, typically in the outer third of the fallopian
tube (ampullar portion).
Oneovum matures per month, and itis capable of fertilizationfor 24–48 hours.
Spermatozoacansurvive for about 48–72 hours.
ertilization window:sexual intercoursemustoccurwithin 72 hours (48 hours
F
before and 24 hours after ovulation).
Ovum
- is surrounded by the zona pellucida and corona radiata, which protect it and
aid its movement into the fallopian tube.
verage ejaculationcontains 50–200 million spermper milliliter(4
A 00 million per
ejaculation).
Spermreaches the cervix in 80 seconds and the fallopiantube within 5 minutes.
, permatozoamove through the cervix, uterus, and fallopian tubesusing flagella
S
and uterine contractions.
apacitationisa final process that sperm must undergoto be ready for
C
fertilization.This process, which happens as thesperm move toward the ovum,
consists of changes in the plasma membrane of the sperm head, which reveal
the sperm-binding receptor sites.
fter sperm penetration, theovum and sperm chromosomesfuse,forming a
A
zygote with 46 chromosomes.
If anX-carrying spermatozoonenters the ovum, theresulting child will have two X
chromosomes and will be female (XX). If aY-carryingspermatozoonfertilized the
ovum, theresulting child will have an X and a Y chromosomeand will be male (XY).
IMPLANTATION
fter fertilization, thezygote migrates to the uterusover 3–4 days, aided by
A
fallopian tube contractions.
itotic cell division (cleavage) begins, and by thetime it reaches the uterus, the
M
zygote consists of 16–50 cells and is called a morula.
hemorula continues to multiply,forming a blastocyst,with outer trophoblast cells
T
(future placenta) and inner embryoblast cells (future embryo).
Implantation occurs 8–10 days after fertilizationwhen the blastocyst attaches to
the uterine endometrium.
heprocess of implantationincludesapposition (contact),adhesion (attachment),
T
and invasion (settling into uterine folds).
Implantationusually happens high in the uterus;lowimplantation(p
lacenta
previa) can cause complications during birth.
Once implanted, thezygote is termed an embryo.
MBRYONIC AND FETAL STRUCTURES
E
The Decidua
- After fertilization, the corpus luteum continues to function due to human
chorionic gonadotropin (hCG) secreted by trophoblast cells.
- The uterine endometrium grows in thickness and vascularity instead of
sloughing off, and is now termed the decidua.
- The decidua has three parts:
, Decidua basalis (below the embryo),
●
● Decidua capsularis (encapsulates the trophoblast),
● Decidua vera (the remaining uterine lining).
- As the embryo grows, it pushes the decidua capsularis until it contacts the
decidua vera, eventually fusing.
- At birth, the entire inner uterine lining is stripped away, increasing the risk of
hemorrhage and infection.
Chorionic Villi
- Once implantation is achieved, the trophoblastic layer of cells of the blastocyst
begins to mature rapidly. As early as the 11th or 12th day, miniature villi, or
probing “fingers,” termed chorionic villi, reach out from the single layer of cells
into the uterine endometrium.
The Placenta
- The placenta (Latin for pancake, which is descriptiveof its size and
appearance at term), arises out of trophoblast tissue.
- Itserves as the fetal lungs, kidneys, and gastrointestinaltract and as a
separate endocrine organ throughout pregnancy.
Its growth parallels that of the fetus, growing from a few identifiable cells at
the beginning of pregnancy to an organ 15 to 20 cm in diameter and 2 to 3 cm
in depth at term.
- It covers about half the surface area of the internal uterus.
URPOSES OF THE PLACENTA
P
Circulation:
- By the12th day of pregnancy,maternal blood beginscollecting in the
intervillous spaces surrounding the chorionic villi.By the3rd week, oxygen
and nutrients like glucose, amino acids, and water pass from maternal blood
to the villi capillaries, which then transport them to the embryo.There is no
direct blood exchange between mother and embryo; instead,nutrients are
transferred via selective osmosis through the chorionic villi. Despite this
barrier,fetal cells and enzymes like alpha-fetoproteincan cross, as well as
most drugs,which is why pregnant women should avoidnonessential
drugs.
Endocrine Function:
- Aside fromserving as the source of oxygen and nutrientsfor the
fetus, thesyncytial (outer) layer of the chorionicvillidevelops into a separate,
important
hormone-producing system.
● Human Chorionic Gonadotropin:The hormone hCGis detectable
in maternal blood and urine shortly after implantation and is used
in pregnancy tests, thoughfalse negatives may occurbefore or after
