Pregancy
The first or primary ovum is released from the ovarian follicle, divides and is expelled into the peritoneal cavity
The cilia of the fallopian tube moves the ovum into one of the two tubes
During this phase, one partner of the 23 paired chromosomes is lost, leaving 23 unpaired chromosomes
Through a series of subsequent divisions, a mature ovum with 23 unpaired chromosomes is produced
One of the unpaired chromosomes is always a female chromosome, called an X chromosome
If this X chromosome is combined with a sperm carrying an X chromosome a female child is formed
If this X chromosome is combined with a sperm carrying a Y chromosome a male child is formed
Fertilization generally occurs soon after the ovum enters the fallopian tube
It takes approximately 3-5 days to move the fertilized ovum the length of the fallopian tube to the uterus
This time frame allows for several more cell divisions to take place before the ovum, now called a zygote, enters the uterine cavity
It takes the zygote an additional 2-4 days to implant itself into the endometrium
Total elapsed time from ovulation is ~ 7 days
After implantation, enzymes begin to digest and liquefy the endometrium to a mass of cells to support the nutritional needs of the growing zygote
A cavity develops within this mass of cells and the embryo is formed and begins to grow along a wall of the cavity
Shortly thereafter, other cells proliferate quickly forming the placenta and membranes
Pregnancy has occurred and, ideally, will develop into a full-term fetus
The Placenta
In the later part of the sexual cycle, the hormone progesterone is secreted stimulating the endometrial cells
Stimulation of these cells produces vast amounts of glycogen, proteins, lipids and some minerals
Purpose is to provide support should a zygote, the fertilized ovum, be implanted
Zygote is implanted and embryo formed
The endometrial cells become more engorged storing even more nutrients
Cells are now known as decidual cells with the total group or mass of cells referred to as the decidua
The decidua is the only form of nutrition available to the embryo for growth and development during the first week following implantation
Despite development of the placenta, the embryo continues to receive the majority of its nutrition from the decidua for 8-12 weeks
The placenta supplies a small amount of the growing embryo's nutrition beginning about 16 days after fertilization
In the early months of pregnancy, the placenta is small and thick making it only slightly permeable
As the pregnancy progresses the placenta grows and thins sufficiently making it easily permeable
In the last month or two of pregnancy, the placenta deteriorates from age--again becoming thickened and less permeable
Minimum distance between maternal blood and fetal blood is 3.5 microns
This is 10 times the distance across the alveolar membrane of the lungs
Despite this distance, many nutrients, as well as other substances, pass through the placenta by diffusion in the same manner of diffusion used elsewhere in the body
At about 16 days after fertilization blood begins to flow through the placenta
Fetal Blood Flow -
The fetus' blood flows through 2 umbilical arteries---
To the capillaries of the placenta's villi where nutrients and oxygen are picked up and CO2 and waste products are given off---
Maternal Blood Flow -
Simultaneously, the mother's blood flows through the uterine and ovarian arteries---
Into the arcuate, radial and spiral arteries surrounding the placental villi where it gives off nutrients and oxygen and picks up CO2 and waste products---
Then back to the mother via the uterine and ovarian veins
Major functions of the placenta
Diffusion of nutrients from the mother's blood to the fetus' blood
Diffusion of excretory products from the fetus' blood to the mother's blood
Amniotic Fluid
Normal volume of amniotic fluid is between 500-1000 mLs
Part of amniotic fluid is produced from fetus' urine excretion
Volume of amniotic fluid increases when fetal urine increases and decreases when fetal urine decreases or there is no urinary output
Likewise, a certain amount of amniotic fluid is absorbed by the fetus' GI tract and lungs
Water in amniotic fluid is completely replaced once every 3 hours
Electrolytes sodium and potassium are replaced every 15 hours
