Overview of normal development of the fetus

Overview of normal development of the fetus with associated clinical significance

Objectives
to review the clinical aspect associated with normal development the fetus and their significance so the student become well acquainted with clinical signs and symptoms of pregnancy

IMPLANTATION AND EARLY DEVELOPMENT OF OVUM

The ova is fertilized in the ampullary region of the tube
After 24 rest starts to divide rapidly foring morula and blastocyst
At day 8-9 it reaches the endometrial cavity
There implantation may occur any where
The endometrium is re named as decidua of which there are 3 types
Decidua baslis
Decidua capsularis
And decidua parietalis
Obliteration of the endometrial cavity occurs when deciua capsularis fuse with decidua parietalis





Early implantation stage
The chorion is a specia tissue which starts to develop also surrounding the fetal sac
The part surrounding the amniotic cavity is called chorion
The part bellow the fetus is called chorion frondosum
The last is the part which is responsible for the placental development
Chorion is composed of 2 layers
Inner of columnar cells which gradully degenerate ; called cytoblast
The outer is a mass of cytoplasm and huge number of nuclei called syncitioblast
The later plays important role in development of the placenta
A heap of cells appears in the blastocyst called fetal pole from which fetus develop

Hormonal changes

Once the placenta is formed even before full maturity starts to secret 4 hormones
1 estrogen- esteriol
Progesterone
Human placental lactogen which ensures good supply of nutrients to the fetus
HCG or human chorionic gonadotrophin which reaches maximum level at 6-8 weeks
And disappear at 16-18 weeks
It supposrt corpus luteum of pregnancy until placenta produce its own amount of progesterone at 12 weeks of gestation

While changes in the blastocyst occur

Inner cell mass starts to form ectoderm and endoderm
This primitive fetus undergoes folding so the endoderm lies inner ward while ectoderm lies outer hood
Endoderm forms all GIT tract while ectoderm forms skin and nervous system
In between a layer of mesenchymal tissue start to appear through which cavities appears which becomes peritoneal cavity and pleural cavity
The yolk sac has no role in mammalian and usually degenerate early unlike in other vertebrates

DEVELOPMENTAL EVENTS OF THE FIRST TRIMESTER

The fetus in the first 12 weeks is highly susceptible to toxins, drugs, radiations and even hormonal insufficiency and
many cycles may be passed without the mother aware she is pregnant
At the stage where inner cell mass undergoes primary differentiation to 3 layers huge changes occur in which at 10 weeks full adult organs are well developed
Brain records of encephalography is perceivable at 11 weeks
HCG starts to appear in blood tests at 5 days after implantation while 2 weeks in urine after implantation
Morning sickness occurs virtually in all women yet with great variability in severity
Embryo changes its name to fetus when fully developed head ad lower legs develop usually 59 day after fertilization corresponding to 30 mm in length

DEVELOPMENTAL EVENTS OF THE FIRST TRIMESTER

Fetal heart rate may be shown first at 6 weeks after fertilization by trans abdominal U/S
While by vaginal U/S scan fetal heart rate may shown as early as 4 weeks after fertilization due to the higher resolution of the U/S device
The primary areola becomes deeply stained while a second areola starts to appear surrounding the primary and disappear after 6 weeks after dekiver
In addition special tubercle like projections starts to appear in the primary areola called Montgomery’s tubercle
They correlates highly pregnancy and this may help the physician to differentiate between many causes of acute abdomen with ectopic pregnancy like twisted ovarian cyst or appendicitis
In addition a dark shaped line appears centrally bellow the umbilcus called linea nigra

Normal development in the second trimester

The uterus become palpable per abdomen suprapubic
Fetal quickening may be fetal 18-20 weeks of gestation
The mother weight starts to increase throughout pregnancy average=12 kg
Fetal weight bypass placental weight at 16 weeks
Physiological anemia starts to appear from hemo dilution
The breast may secret a fluid which is clear and yellow in color called colostrums
Fetal heart rate may be audible by the old PINARD stethoscope
The fetus starts to show response to acoustic stimuli
The gesture of mother starts to change during standing due to lumbar lordosis
HCG should become negative after 20 weeks
Alpha fetoprotein starts to appear in the blood
Maternal cardiac output increase with increase in plasma volume by 40 percent and RBC 20 %
All clotting factors also increase except 11 ad 13 during the rest of pregnancy

Fetal changes in the third trimester

Lanugo hair and vernix caseosa
The pregnancy complication like hypertension
After 37 weeks the head becomes presenting
Increase in hemo dilution and cardiac output
Blood supply to placenta 20%
The pelvic maternal joints start to loose and hyper mobility
In male fetal testis descend to scrotum via gubernaculums
Cervix tend to become soft in texture
Vaginal appear blue in examination
Colostrums ,may increase toward delivery and contains IgG
Milk starts to secrets 2 days after delivery
Lordosis and waddling movement is a rule
Amniotic fluid reaches maximum at 35 weeks and reduce there after
Involuntary loss of urine may occur
Labor cause in human is unknown , fetus may pay role in its onset