Chapter 8 - Third Month to Birth: The Fetus and Placenta Flashcards
What characterize the fetal period?
- Maturation of tissues and organs
- Rapid growth of the body
When is the fetal period?
The period from the begining of 9th week to birth
Sitting hight
- Crown-rump lenght (CRL)
- Lenght of the fetus
Standing hight
- Crown-heel lenght
- Lenght of the fetus
How long is the pregnancy?
- 280 days, or 40 weeks after the normal mensturation
- 266 days or 38 weeks after fertilization
When does the head constituet 50% of the CRL?
At the beginning of the 3rd month
When is the head 1/3 of the CHL?
By the beginning of the 5th month
When is the head 1/4 of the CHL?
At the time of birth
When does the face become more human looking?
During the 3rd month
When is the primary ossification center are present in the long bones and skull?
By the 12th week
When does the external ganglia develop to such a degree that the sex of the fetus can be determined by external examination (ultrasound)?
By the 12th week
When can reflex activity be avoked in aborted fetuses, indicating muscular activity?
At the end of the 3rd month
When does the fetus lengthen rapidly?
During the 4th and 5th months
First half of intrauterine life
- CRL 15 cm = half of the total length of a new born
- Over 500 g
- Lanugo hair
- Eye browns and head hair
When can the movements of the fetus be felt by the mother?
During the 5th month
Second half of intrauterine life
- Wight will increase, especially during the last 2,5 months..(3,200 g)
Premature
Fetuses born much earlier
Postmature
Fetuses born later
Placenta
The organ that facilitates nutrient and gas exchange between the maternal and fetal compartments
When does the major changes in the placenta occure?
As it begins the 9th week
Where does the fetal components of the placenta derive from?
- Derive from the trophoplast and extraembryonic mesoderm (The chorionic plate).
- from the summary: Derive from chorion frondosum or villous chorion
Where does the maternal components of the placenta derive from?
- uterine endometrium
- From the summary: the maternal portion derive from decidua basalis
Where is the intervillous lakes of maternal blood?
It is in the space between the chorionic and decidual plates
Where can we find the villous tree(fetal tissue)?
They grow into the maternal blood lakes and are bathed in them
Is the fetal circulation separated from the maternal circulation and how?
Yes, by:
- A syncytial membrane (a chorion derivate)
- Endothelial cells form fetal capillaries
The human placenta is of the…
hemochorial type
How much maternal blood is it in the intervillous lake?
150mL = 0,150L
How often is the maternal blood renewed?
3-4 times a minute
Main functions of the placenta:
- Exchange of gases
- Exchange of nutrients and electrolytes
- Transmission of maternal antibodies, providing the fetus with passive immunity
- Production of hormones, such as progesterone, estradiol, and estrogen. in addition: hCG and somatomammotropin.
- Detoxification of some drugs
The amnion
- Large sac containing amniotic fluid, produced by amniotic cells but is derived primarly from maternal blood.
- The fetus is suspended in it by its umbilical cord.
- The fetus swallows amniotic fluid, which is absorbed through its gut and cleared by the placenta.
- The fetus adds urine to the amniotic fluid, but this is mostly water.
The fluid in the amnion
- Absorbs jolts
- Allows for fetal movements
- Prevents adherence of the embryo to surrounding tissues.
An excessive amount of amniotic fluid (hydramnios) is associated with..
Anencephaly and esophageal atresia
An insufficient amount (oligohydramnios) is related to….
Renal agenesis
The umbilical cord, surrounded by the amnion, contains:
- Two umbilical arteries
- One umbilical vein
- Wharton’s jelly, which serves as a protective cushion for the vessels.
Dizygotic, or fraternal, twins
Two thirds of twins are dizygotic, or fraternal; they have two amnions, two chorions, and two placentas, which sometimes are fused.
Monozygotic twins
Monozygotic twins usually have two amnions, one chorion, and one placenta.
Conjoined twins
In cases of conjoined twins, in which the fetuses are not entirely split from each other, there is one amnion, one chorion, and one placenta.
Signals initiating parturition (birth) are not clear, but preparation for labor usually begins between 34 and 38 weeks.
Labor itself consists of three stages:
- Effacement and dilatation of the cervix
- Delivery of the fetus
- Delivery of the placenta and fetal membranes.
What characterize the trophoblast by the second month?
Great number of secondary and tertiary villi
