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Gestation Period

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-Comprises 3 trimesters
-In first trimester individual starts out as a zygote, then morula, blastocyst
-after implantation, is called an embryo
-embryonic phase of development lasts from fertilization until the 8th week of gestation, when it becomes a fetus
-by day 35 the heart is beating, and eye and limb buds are present

Placenta and Umbilical Cord

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-Placenta forms during 3rd month
-Develops from embryonic chorion and maternal uterine tissue
-Site of nutrient, gas and waste exchange
-Secretes hormones that maintain pregnancy
-Barrier to microorganisms, except some viruses
-AIDS, measles, chickenpox, poliomyelitis, encephalitis
-not a barrier to drugs such as alcohol
-The umbilical cord, containing two umbilical arteries and one umbilical vein connects the embryo and the placenta
-At birth, the placenta detaches from the uterus (afterbirth)

Hormone Blood Levels

-Human chorionic gonadotropin (hCG) produced by the chorion is less important after 4 months, because the corpus luteum no longer must be stimulated
-The placenta takes over the steroid hormone secretions of the corpus lutem


Rescues the corpus luteum from degeneration until the 3rd or 4th month of pregnancy

Progesterone Estrogens

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1. maintain endometrium of uterus during pregnancy
2. help prepare mammary glands for lactation
3. prepare mother's body for birth of baby


1. increases flexibility of pubic symphysis
2. helps dilate uterine cervix during labor


1. Helps prepare mammary glands for lactation
2. Enhances growth by increasing protein synthesis
3. Decreases glucose use and increases fatty acid use for ATP production

Corticotropin releasing hormone

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1. Establishes the timing of birth
2. Increases secretion of cortisol

Primary Germ Layers

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In between the yolk sac and the amniotic cavity is the embryonic disc, which gives rise to the primary germ layers:
-Arises from the layering of cells that occurs during gastrulation day 15
Organogenesis is the formation of the organs starts 4th week


All nervous tissue
Epidermis of skin


Cartilage, bone and connective tissue
Skeletal, cardiac and smooth muscle tissue
Blood and lymphatic vessels
Epithelium of gonads


Epithelium of digestive tract
Epithelium of respiratory system
Epithelium of thyroid, liver and pancreas
Epithelium of bladder

Menstrual Phase

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-Menstruation lasts for 5 days
-First day is considered beginning of 28 day cycle
-In Ovary
-a cohort of follicles 20 that began to develop 6 days before beginning to secrete estrogen
-Fluid is the filling the antrum from granulosa cells
In uterus:
-declining levels of progesterone causes spiral arteries to constrict and glandular tissue degenerates
-stratum functionalis layer is sloughed off along with 50-150 ml of blood

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