Chapter 6: Conception and Prenatal Development Flashcards
What is implantation?
Implantation of the fertilized ovum (zygote) in the uterine endometrium.
Organogenesis
Organs are formed primarily during the first 8 weeks of pregnancy.
Teratogens
an agent or factor that causes malformation of an embryo
Fertilization
Occurs when one spermatozoon enters the ovum and two nuclei containing parents’ chromosomes merge.
Where does fertilization usually occur?
In the distal third of the Fallopian tubes. (Aka ampulla)
Entry of one spermatozoon into the ovum has three results:
- Zona reaction - changes in the zona pellucida surrounding the ovum prevent other sperm from getting in.
- Cell membranes of the ovum and sperm fuse and break down, allowing the contents of the sperm head to enter the cytoplasm of the ovum.
- Ovum completes meiosis (ovum now has 23 chromosomes and expulsion of 2nd polar body)
Three periods of fetal growth?
- Pre-embryonic
- Embryonic
- Fetal
What happens during the pre-embryonic period?
- Moment the ovum is fertilized.
- Initiation of cell division occurs.
- Entry of the zygote into the uterus
When does the pre-embryonic period occur?
The first 14 days of development.
Pre-embryonic period: Cell Division
- Zygote divides into 2, then 4, then 8 cells and so on, until the 16-cell stage where it becomes the morula (12-16 cells).
- Outer cells of morula secrete fluid, forming a blastocyst (a sac of cells with an inner cell mass placed off center within the sac)
- The inner cell mass develops into the fetus.
- The outer layer of cells develop as the placenta and fetal membranes.
When does the zygote enter the uterus?
Around the fourth day after conception is when the blastocyst, which contains approximately 100 cells, enters the uterus.
Pre-embryonic period: Entry of the zygote into the uterus
- After it enters the uterus, the blastocyst lingers in the uterus another 2-4 days before beginning implantation.
- The endometrium is now called the decidua, and is in a secretory phase of the reproductive cycle.
Decidua
Endometrium of a pregnant woman; when it thickens to prepare for pregnancy.
When does implantation occur?
6-10 days after conception*
Implantation is complete by
The second week after fertilization.
Why is the location and timing of implantation critical for continued development?
Requires a continuing supply of estrogen and progesterone to maintain decidua in the secretory phase.
What signals a woman’s body that pregnancy has begun?
The zygote secretes human chorionic gonatropin (hCG) to signal the woman’s body that pregnancy has begun.
What does hCG do?
Causes the corpus luteum to persist and continue secretion of estrogen and progesterone until the placenta takes over this function.
Where does normal implantation occur?
Upper uterus, slightly more often on the posterior wall.
The upper uterus is the best area for implantation and placental development because of 3 reasons:
- Richly supplied with blood for optimal fetal gas exchange, nutrition and waste elimination.
- Uterine lining is thick, preventing the placenta from attaching to deeply into the uterine muscle (this can facilitate easy expulsion of placenta after full-term birth)
- Limits blood loss after birth (strong interlacing muscle fibers in this area compress open endometrial vessels after the placenta detaches)
When does the conceptus become fully embedded w/ in the mother’s uterine decidua?
By 10 days*
Nourishment of the fetus
Nutritive Fluid passes to the embryo by diffusion in early stage d/t circulatory system not being established yet.
……
When does the embryonic period occur?
In weeks 3-8.
Embryonic Period: Teratogens
Structures are vulnerable to damage by teratogens because these structures are developing rapidly.
When does the fetal stage occur?
In week 9 when the embryo turns into a fetus.
When is a fetus considered viable?
By week 24. This is when the fetus will be able to survive delivery.
When does production of surfactant occur?
Begins at approximately 20 weeks but does not reach levels that increase the likelihood of survival outside the uterus until 28-30 weeks of gestation.
What interventions can be done to reduce the respiratory distress of prematurity?
- Artificial surfactant may be given.
2. Maternal corticosteroid such as Bethamethsone
How do maternal corticosteroids such as bethamethsone help a preterm fetus with/at risk for respiratory distress?
May be given before birth to accelerate surfactant production in the preterm fetus likely to be born before completion of 34 weeks of gestation.
Why is a fetus born before 24 less likely to survive d/t inadequate gas exchange?
Because the capillary network around the alveoli is still very immature before 24 weeks.
A fetus is considered full term between what weeks?
38-40 weeks (book says 38 but she says 37)
What happens during the embryonic period?
- Basic structures of all major body organs are completed.
- Body cells are differentiated and specialized.
What can be done to reduce the incidence of neural tube defects during organogenesis?
Taking vitamins and folic acid
Two most common NTD’s include
Spina bifida and anencephaly
Pregnancy Medication Classifications
Category A Category B (i.e tylenol) Category C (i.e antibiotics, antifungals) Category D Category X
Category A Drugs
Safest.
No evidence of risk to fetus exists.
Dicligis
For N/V
Category A
Category C Drugs
Considered OK
Category D Drugs
- Not so safe.
- Positive evidence of human fetal risk exists based on adverse reaction data, but potential benefits may warrant use of the drug in pregnant women despite fetal risks.
Category X Drugs
- Recommended to women to terminate pregnancy.
- Positive evidence of human fetal risk.
Isotretinoin (Accutane)
- A vitamin A derivative for acne.
- Pregancy category: X
- Should not be taken during pregnancy because it causes fetal defects.
- Excessive intake causes spontaneous abortions or serious fetal defects.
Teratogens include
- Medications
- Radiation (i.e X-rays, CAT scan)
- Hot tube use (no longer than 20 minutes)
- Live vaccines (i.e rubella vaccine)
- Substance abuse
- Alcohol
Teratogens pose concerns for
Spontaneous abortions
Possible CNS defects
Failure of neural tube closure
Intrauterine Growth Restriction
- Impaired placental perfusion (i.e HTN, vasoconstriction) decreases supply of glucose and oxygen delivered to the fetus.
- As a result, the infant is likely to be small for gestational age.
Two types of “ages”
- Fertalization age
- Gestational age
Fertilization age
36-40 weeks (2 weeks shorter than gestational)) From conception (conception occurs approximately 2 weeks after first day of last menstrual period)
