General embryology

GENERAL EMBRYOLOGY

The development (formation) of a new individual begins by the union of 2 cells,one from the male (Sperm) & one from the female ( Ovum).The union of the 2 cells is by fertilization tforming Zygot ( fertilized ovum).Each of the sperm & ovum has haploid number chromosomes & after fertilization the Zygot will have Diploid number of chromosomes.
Pregnancy last for 280 days(i.e ten lunar months from the time of fertilization)or 266 days from the time of implantation of the Blastocyst.The length of pregnancy includes 3 stages:
1-The zygot stage ( fertilized ovum) lasts for 2 weeks after fertilization.
2-Embryonic stage ,extends from beginning of 3rd week to end of 8th week.
3-Fetal stage,extends from beginning of 9th week to the end of pregnancy.
The general features of each stage are as follows:
1-Zygot stage.All the cells look like each other.
2-Embryonic stage,here the tissues,organs & systems are formed( Organogenesis)
3-Fetal stage.Here the body grows in length ,weight and tissues & organs develops further.

Gametogenesis

It means the formations of sperms (in the testes) by a process known as Spermatogenesis
And mature ovum(in the ovary) by a process known as Oogenesis.Each of the sperm & ovum contains Haploid number of chromosomes.
Spermatogenesis(formation of sperms)includes spermatocytogenesis & spermiogenesis.
In spermatocytogenesis ,the primitive male sex cells (spermatogonium) with 46 chromosomes divides by Mitotic divisions to give rise type A & type B spermatogonium.
Type A spermatogonium remains in contact with the basement membrane of seminiferous tubules of of the testes,while type B spermatogonium continues maturation and is transformed in to Primary spermatocytes( contains 44 + xy),then it undergoes the first meiosis division to give 2 secondary spermatocytes( 23 chromosomes).Each secondary spermatocye inters the second meiosis division to give 2 spermatids(each has 23 chromosomes).The spermatid is not capable to move and not capable to penetrate & furtilzes the ovum unless change its shape in process known as Speriogenesis ,where it undergoes some morphological changes to become capable to move & follws the mature ovum to penetrates & fertilizes it to form Zygot.
The SPERMIOGENESIS includes the following changes:
A-The nucleus of the spermatid form the head of the sperm(contains genetic material)
B-The Golgi apparatus forms the Head cap ( Acrosome ) of the sperm.
C-The Mitochondria will form a sheath around the middle piece of the sperm(energy).
D-The proximal centriole come to lie in the neck & gives rise to the axial filament,which forms the central core of the middle piece& tail of the sperm,while the distal centriole becomes ring shaped forming the terminal ring.
The head of the sperm attaches itself to the Sertoli cell to get nourishment,then discharged during Ejaculation in the seminal fluid(from sex glands as prostate,seminal vesicles & bulbourethral glands).The volume of seminal fluid at Ejaculation is 3-5 cc which contains about 100 million of sperms in each cc.The sperms are genetically of 2 types ,one carries X- chromosomes,while the other carries Y- chromosome.On the other hand the Ovum are almost always of one type carries X- chromosomes.


OOGENESIS
It is a cyclic process which takes place every lunar month(28 days)under the influence of follicle stimulating hormone (F.S.H)from anterior lobe of pituitary gland.In Oogenesis the following events takes place:
1-Formation of Graafian follicle
2-Ovulation.
3-Formation of Corpus Luteum.
It starts from puberty(11-14) years old(Menarch) and ends at Menopause(45-50) years old..The Graafian follicle is formed by maturation of the Primordial follicle(consists of a primary oocyte surrounded by a layer of follicle cells).The Primary Oocyte undergoes the first Meiosis division(not completed unless ovulation takes place) giving secondary Oocyte plus first polar body.The follicular cells increases in number & aquires granules in the follicular cytoplasm ,and thus is called Granulosa cells(Zona granulosa),then a cavity appears between the granulosa cells known as follicular antrum and filled with fluid.This fluid separates the Granulosa cells into two parts:
A-Stratum Granulosum (outer layer)
B-Cumulus Oophorous on the inner side of the Graafian follicle & surrounding the oocyte(the oocyte is separated from them by Zona Pellucida).
All the follicle then becomes surrounded by Theca ,which consists of 2 layers:
1-Theca externa of fibrous layer.
2-Theca interna which secretes Estrogen hormone.
The follicular stage is under the influence of the F.S.H from pituitary gland.

OVULATION

Means rupture of the Mature Graafian follicle& the release of mature ovum.The ovulation takes place at day 14+1 of the ovarian cycle.It is under the influence of Luitinizing hormone (L.H) from pituiray gland too to induces ovulation ,which causes by the following factors:
1-Increases in the intrafollicular pressure.
2-The influence of L.H
On ovulation ,the oocyte starts the second meiosis division,which is not completed until fertilization of the Ovum is completed.
3-The walls of the G.F becomes very thin .
4-The cells of Cumulous oopharous become loosened.
Thus the G.F rupture and the ovum is released(now the first meiosis division is completed ) and secondary oocyte will inter immediately the second meiosis division,which is not completed until fertilization of the Ovum takes place.Once fertilization is completed ,the second meiosis division is completed.The result of oogenesis is to get a single mature Ovum & 3 polar bodies.
The Luteal Phase
Uppon the rupture of the mature G.F and the release of the ovum,what remains from the GF becomes known as the Corpus Luteum(in the ovary) due to :
1-The wall of the ruptured GF collapses & becomes folded.
2-The granulosa cells becomes filled with yellow pigment known as Lutein.
3-The Theca Externa forms the capsule of the Corpus Luteum.
4-The Theca Interna provides the Corpus Luteum with vascular C.T .
The corpus luteum secretes Progesterone which is responsible for the Secretory stage ( post ovulatory) in the Endometrium to prepare it for implantation
The fate of Corpus Luteum:
A-If the Corpus Ovum is not fertilized ,the Corpus Luteum degenerates & is converted into fibrous tissue called Corpus Albicans after 14 days after ovulation.
B-If the Ovum is being fertilized ,then the corpus luteum persists & increases in size to increase the production of progesterone which is essential for the maintenance of pregnancy.This is known as Corpus Luteum of pregnancy,which persists until the 4th month of pregnancy,when it is going to regress & the secretion of progesterone is going to be the function of the placenta.


Fertilization
It takes place in the ampullary part of the fallopian tube,where fusion of the male gamete( Sperm) & the female gamete ( Ovum) takes place as follows:
The head of the sperm only passes through the follicular cells of the Corona radiate (that surrounds the ovum ) ,then it penetrates the Zona pellucida with the aid of proteolytic enzyme secreted by the acrosomal cap of the sperm& finally the head becomes in the cytoplasm of the Ovum.Then the head of the sperm ( male pronucleus) fuses with the nucleus of the Ovum ( female pronucleus) to form the nucleus of the Zygot ( Fertilized ovum ).Thus the zygot is now becomes DIPLOID .The fertilization results in the followings:
1-Restoration of the Diploid number of chromosomes.
2-Determination of the Sex of the new organism ( going to be male or female).
3-Initiation of repeated mitotic divisions ( Cleavage).
4-Formation of the Zygot.

THE MENSTRUAL CYCLE

Is the cyclic changes in the endometrium of the uterus which occurs every lunar month(28 days) in response to cyclic changes in the ovary.It starts from Menarch (Puberty at 11-13 years)& ends at Menapause(45-50 years).It includes 3 stages:
1-Menstrual stage: Where bleeding of about 30-60 ml of blood is discharged per vagina due to lowering of blood progesterone level caused by degeneration of Corpus Luteum.It last for 3-5 days& the discharge consists of :
a-Clotted blood.
b-Dead( not fertilized) ovum .
c-Sheded pieces of endometrium( includes stratum compactum & spongiosum)
2-Proliferative stage,which last for 7-10 days after bleeding has stopped,it coincides with the follicular stage of the Ovarian cycle& is due to Estrogen secretion from follicular cells of the growing GF.During this stage the stratum basalis of endometrium which remains after menstruation will start to reform the stratum compactum & spongiosum that sheded with the bleeding,thus:
a-Endometrium may reach 2-3 mm in thickness just before ovulation.
b-The arterioles grow & become coiled.
c-The glands increases in length & becomes tortous ( coiled).
3- Secretory stage,which starts immediately after ovulation& is due to progesterone secretion from the Corpus luteum.It lasts for 10-14 days& coincides with the luteal phase of the ovary,thus:
a-Endometrium becomes 5-8 mm in thickness & consists of 3 stratum ,compactum ,spobgiosum and basalis.Thus the Endometrium is now ready to receive the fertilized ovum (Blastocyst ) for implantation.
b-The glands becomes more tortous & secretes mucin& glycogen ( uterine milk).
c-The arteries that supply compactum & spongiosum are called spiral arteries,which becomes very tortous & tall,while arteries supplying the basal layer iare called as Basal arteries( short & straight).
4-Premenstrual stage: if by the end of secretory stage no fertilized ovum reaches the endometrium,then progesterone secretion will reduce & stop.Thus the spiral arteries& tortous glands will regress& endometrial mucosa starts to reduce in thickness,hence stratum compactum & spongiosum prepare themselves for sheding.This stage lasts about 2 days ,just prior to the menstruation phase.


CLEAVAGE
A term which means repeated mitotic divisions of the Zygot,it occurs rapid & in succession.It starts immediately after fertilization(also known as segmentation).
The human Zygot becomes 2 cell stage 24-36 hours pos-fertilization, 4 cell stage 40-48 hous post-fertilization,then early Morula ( 16 cell stage )at 3rd day. looks like a mulberry,which is a solid ball(still surrounded by Zona pellucida.At day 4 it becomes late Morula ( 32 cell stage).It is still solid & reaches the uterine cavity& obtains its nutrition now from endometrial secretion ( uterine milk).It then become 64 cell stage ( early Blastula) which starts to loose its Zona pellucida & try to stick itself to endometrial mucosa of the posterior surface of the fundic region& continue divisions till it becomes of 107 cells stage ( late Blastocyst).It has the following features:
1-Bigger in size with relatively large cavity due to fluid collection.
2-It is 107 cell which arrange as inner cell mass of 8 cells known as Embryoblast
& outer cells on the circumference of the Blastocyst of 99 cells known as Trophoblast layer ( future placenta).
3-Lost the Zona pellucida completely to facilitate implantation.
4-It has 2 p0les ,an Embryonic pole ( where inner cell mass is present),while the other side is known as Ab-Embryonic pole.
5-Starts to open a very tiny gap in the endometrial mucosa in order to inter through it & implants itself there.
As it loses completely the Zona pellucida ,the trophoblast becomes exposed,thus becomes able to stick& eat up other cells ( Endometrial cells) to invade itself in to the mucosa with the aid of proteolytic enzyme secreted by the trophoblastic cells at the site of the embryonic pole.

IMPLANTATION

Is the embedding of the Blastula into the endometrial mucosa,usually in the upper part of posterior wall of the fundic region.It starts at day 6 post-fertilization& is completed at day 12.The late Blastocyst loses zona pellucida,then stick itself to the endometrium & with the aid of proteolytic enzyme secreted by the outer layer of the trophoblast,a tiny opening is created and the blastocyst starts to invade & burrow itself into the endometrium.This process takes about a week ( from day 6-12 postfertilization)&once embedding is completed,the tiny opening in the endometrium is closed by a fibrin coagulum,
Which is finally replaced by epithelium of the endometrial mucosa.
Abnormal Implantation sites
A-Intrauterine which is known as placenta praevia,where implantation is near the cervix & according to the exact site of implantation,we can see 3 named placenta praevia,these are:
1-Placenta praevia lateralis,where implantation is just above the internal oss.
2-Placenta praevia centralis,implanta around the internal oss&closes it.
3-Placenta praevia marginalis,where margin of placenta reaches above the
Internal oss of the cervix.
B-Extrauterine implantation sites: includes the followings:
1-Tubal pregnancy,where implantation takes place in the ampullary region of the uterine tube& is terminated at about 6 weeks of pregnancy as ABORTION.
2-Interstitial implantation i.e in the interstitial part of the uterine tube.
3-Ovarian implantation I .e on the surface of the Ovary.
4-Intraabdominal implantation on the peritoneum of the abdomen.
5-Pelvic implantation,where implantation in rectouterine pouch ( of Douglass)


THE THIRD WEEK OF DEVELOPMENT

1-The Bilaminar Embryonic disc is converted in to Trilaminar disc by the appearance of Mesoderm between the Ectoderm & Endoderm.
2-The Trophoblast shows 3 changes during 3rd week as primary,secondary & tertiary velli.The primary villous orms by a ciore of cytotrophoblast covers by syncytrophoblast layer.Secondary velli forms by a core of Mesoderm covers by cytotrophoblast ,then syncytiotrophoblast.The Tertiary velli shows fetal capillaries in the core of extraembryonic mesoderm.
3-The Embryonic Disc shows the appearance of 3 important structures as:
a-The primitive streak b-Notochord c-Neural tube
The primitive streak
Is a structure starts to appear in the mid line of the embryonic disc,starts to appear at day 14(end of 2nd wk)& becomes very clear at days 14-15.It is a narrow groove having a slightly bulging areas on either side.The cells of the streak are in fact a modified ectodermal cells
The appearance & formation of primitive streak proceeds the formation of the intraembryonic mesoderm,where the ectodermal cells in the region of primitive streak starts to migrate in between the already present ectoderm & endoderm & creeps in all directions except in 2 areas( the buccopharyngeal cloacal membranes).
The movement of the newly formed mesoderm( i.e modified ectodermal cells) anteriorly occurs in characteristic way,where they migrate anteriorly on either sides of the mid line & meets at the Prochordal plate,thus the mid line area between the Hensen's node of the primitive streak & prochordal plate remains as a bilaminar in nature( i.e doesn’t contain any mesoderm layer)After its formation mesoderm will differentiate into 3 named parts as follows:
A-Paraxial mesoderm,is most medially located & becomes segmented into blocks( Somites) of about 42-44 segments.This will form the skeleton ,skeletal muscles & the skin.
B-Lateral plate mesoderm, it crosses from side to side cranial to buccopharyngeal membrane to form the cardiogenic area.In this part of the mesoderm ,a U shaped structure cavity appears to form the future pleural & peritoneal cavities
C-Intermediate mesoderm,lies between paraxial & lateral plate mesoderm ,it is cord klike known as Nephrogenic cord which gives rise to urogenital system.At day 19 the Embryonic Disc becomes completely as Trilaminar embryonic disc..

THE NOTOCHORD

It is an important structure which develops during the 3rd wk& is considered as a supporting element for the growing fleshy Embryo and is being used as an Axial skeleton(Temporary) until the vertebral colum is formed.
It starts formation as a notochordal(Head) process,that develops by invagination & proliferation of some Ectodermal cells at the sides of the Henson's (primitive) node.It extends as a middle structure between the ectoderm & endoderm extending from Henson's node to the prochordal plate.The notochordal process can be seen very clealy when the Embryo is 17 days old& on day 18 a canal appear
In the notochordal process known as neurenteric canal.This canal will form temporary a connection between the Amniotic cavity & primary yolk sac& when the notochordal process fuses with the endoderm,the notochordal-endodermal plate is seen.Finally the roof of the notochordal process is folded around its axis to form the notochord after the canal loses its connection with the 2 cavities & the amnion becomes completely separated from the yolk sac again.

THE NEURAL TUBE

At first , a neural plate appears dorsal to the already formed Notochord.This neural plate is a thickened part of the Ectoderm that lies behid the notochord & along each side of the neural plate lies the Neural crest.On day 18 the plate becomes modified to form a median grooved part & 2 lateral raised parts known as Neural folds.The edial ends of the 2 folds are approximated gradually & the groove becomes deeper and deeper & eventually the 2 Neural folds fused together forming the Neural Tube,which becomes just dorsal to the Notochord.
Just on the dorsolateral sides of the Tube we can see the Neural Crest,the cephalic part of the Neural tube becomes expanded & dilated forming the Brain Vesicle(the future brain),while the caudal part differentiate to form spinal cord
THE DERIVATIVES OF THE NEURAL CREST
1-Neurons of Dorsal root ganglion of spinal nerves.
2-Neurons of Sympathetic ganglion.
3-Neurons of sensory ganglia OF CRANIAL NERVES 5TH,7TH,8TH,9TH,&10TH .
4-Shwan cells of the peripheral nerves.
5-Melanoblast of the skin.
6-Chromaffin pigmented tissue.
7-The cells of the Suprarenal medulla( Adrenal medulla).
8-The Pia & Arachnoid layers of the Central Nervous System.


SECOND WEEK DEVELOPMENT
Is being considered as the week of ''TWOS'' and always remember 2 in 2nd wk as:
1-The trophoblast becomes 2 layers syncytiotrophoblast& inner cytotrophoblast.
2-Embryoblast becomes Bilaminar disc ,an outer Ectoderm &innerer Endoderm.
3-Two cavities are formed during this wk an Amniotic cavity & Yolk sac.
At Day 8 the Blastocyst is partially embedded in the Endometrium & the part of Trophoblast that lies over the inner cell mass(Embryonic pole)is differentiated into an outer dark zone which takes the responsibility of implantation by opening a tin y gap in the endometrial mucosa with the aid of proteolytic enzyme(Syncytiotro..)&an inner pale(lighter) zone having a clear cellular boundaries known as Cytotrophoblas which directs the nutrients materials coming to it from the outer Syncytiotrophoblast toward the inner cell mass(Embryoblast).
The EMBRYONIC DISC is differentiated in to what is called as Bilaminar germ disc consists of inner small rounded and polyhedral cells facing the Blastocyst cavity known as Endoderm(Hypoblast)& outer tall columnar layer Ectoderm (Epiblast).
The amniotic cavity starts to form at this time whose roof is formed by Amnioblast( derives from inner surface of cytotrophoblast),while it's floor is the outer surface of the Ectoderm(Epiblast).
DAYS 9 & 10:t of Development
1-Implantation is about to finish & fibrin coagulum appears at implantation site.
2-Developmental changes in the Trophoblast is more progress compared to that of the Embryonic disc which is relatively slow& the trophoblast differentiate into 2 layers surrounding the Blastocyst completely at this time.At the mean time Vacules & lacunae appears in the Syncytiotrophoblast layer,thus this stage is called the Lacunar stage.
3-The Amniotic cavity becomes much larger.
4-Some cells from the Endoderm layer creeps on the inner surface of the cytotrophoblast forming what is called Heuser's membrane which becomes continuous with the edges of the Endoderm& forming the primary yolk sac(it's roof is by Endoderm & it's floor by Heuser's membrane).

Days 11th & 12th Development

1-Implantation is completed at 12th day & fibrin clot is replaced by endometrial cells.
2-The 2 cavities(Amniotic & primary yolk sac) show no definitive changes.
3-The Trophoblast shows the following changes:
a-Lacunae increases in number & size,and some of them communicates & open to other lacunae forming what is called the Trabecular appearance in the Syncytium
b-More erosion of the stromal endothelial layer of the capillaries produces sinusoids,as a result the syncytial lacunae become continuous with sinusoids & maternal blood enters the lacunae thus establishing what is called Utero-placental circulation.
4-Formation of extraembryonic mesoderm & coelomic cavity.The extraembryonic mesoderm is formed from the inner surface of cytotrophoblast& doesn’t share in the formation of any tissue or structure of the embryo.Thus the trophoblast is called now as chorion,while the Blastocyst is called now as Chorionic vesicle.
The extraembryonic mesoderm includes 2 parts as :
a-Somatopleural mesoderm covering the amniotic cavity.
b-Splanchnopleuric membrane covering the yolk sac.
The extraembryonic mesoderm forms the connecting stalk at the site of connection of the embryo with the trophoblast.This stalk will progress latel cn into the umbilical cord..while the 2 parts of extraembryonic mesoderm are separated from each other by the extraembryonic coelom(chorionic cavity).
5-Decidual reaction at the site of attachment of the embryo to the endometrium.The cells become larger in size,polyhedral & storing materials to supply the embryo with nutrient materials.This will progress into what is called as Decidua Basalis which is considered as maternal part of the placenta.


Day 13th
1-The primary chorionic velli form from the trophoblast around the embryonic pole & this will progress into the fetal part of placenta(chorionic frondosum).
2-The major part of the primary yolk sac is pinched off&the remaining part becomes the secondary yolk sac (Definitive yolk sac).
3-Bleeding may occur at the site of implantation& confuses with menstrual bleeding.
4-The connecting stalk becomes more clear at this day.

Day 14th

At this time a localized area of thickening part named the prochordal plate appears in the anterior part of the endoderm layer.At this place the endoderm becomes very adherent to the overlying ectoderm.
Day 14 is the day of prochordal plate& is also the day of bleedings which might confuses the woman about the exact date of her conception ( pregnancy).

THE EMBRYONIC PERIOD

Is the period of Organogenesis,where each of the 3 layers of Trilaminar disc gives rise to a number of tissues& organs.It extends from 3rd-8th weeks…….
The derivatives of the Ectoderm are:
1-Neuroectoderm which gives rise to C.N.S ,, P.N.S ,& autonomic N.S.
2-Sensory epithelium of the ear ,nose & eye.
3-The epidermis of skin & skin appendages as hair,nails,sweat 7sebacous glands.
4-The pituitary gland & Medulla of suprarenal gland ( Adrenal ).
5-The breast & sweat glands.
6-The epithelium of anterior part of Oral cavity & lower part of Anal canal.

The Derivatives of the Mesoderm are :

1-The dermis of the skin & connective tissue.
2-The muscles of trunk & skeleton( except the skull)
1 & 2 form the Paraxial mesoderm which becomes separated from day 20 into Somites.At day 20 the first pair of somites appears,then 3 pairs are added daily till day 35 where they become 42-44 somites ,which are arranged as 4 occipital, 8 cervical,12 thoracic, 5 lumbar,5 sacral & 8-10 coccygeals.
Each somites is differentiated in to 2 parts an inner (ventromedial part) called Sklerotome( forms vertebrae & bones ) & an outer (dorsolateral part) as Dermomytotome( gives muscles & strip of skin).
3-The intermediate Mesoderm ( Nephrogenic cords) gives rise to the Urogenital&accessory glands.
4- Lateral plate Mesoderm will gives rise to the followings:
a-Connective tissues,smooth muscles of viscera & limbs.
b-Blood vessels,blood cells & lymphatic tissues & lymph cells.
c-The Cardiovascular system.
d-The Spleen.
e-The Adrenal cortex.


The Derivatives of The Endoderm are:
1-The epithelial lining of Gastrointestinal tract including liver &pancreas.
2-Thyroid& parathyroid glands + Tonsils.
3-The Tympanic cavity & pharyngotympanic tube.
4-The epithelial lining of the Pharynx.
5-Epithelial lining of Trachea ,Bronchii & lungs.

THE FETAL PERIOD

It extends from the 9th week to the end of pregnancy (40 wks) i.e from beginning of 3rd month until delivery time ( birth of baby).The pregnancy extends for 40 wks from the date of the last menstrual period(L.M.P) which is equivalent to 280 days.
Or 38 wks from the date of fertilization( 266 days).
The length of the fetus can be estimated as Crown-Rump length(CRL)i.e sitting position early in the fetal period ,or Crown-Heel(CHL) i.e standing height later on during the fetal period.The length is expressed in cms,then correlated with age of the fetus.The growth in length is a striking feature in the 3rd-5th month with an a verage of about 5 cm per month,while the increase in weight is a striking feature during the last two month with an a verage increase as 500 grams per month during the last two months.
The features of fetus by the end of the 3rd month ( i.e 12-13 weeks) are:
1-The face appears to be more human looking.
2-The limbs reach their relative length & position in comparison to rest of body.
3-Primary ossification centers can be seen in some long bones& skull.
4-The external genitalia becomes clear& sex can be determined at this time.
5-The head is an enormous part of the fetus& is about half the CRL(sitting ).
The Features at the 5th month are :
1-The head is about one third the crown-heel length(CHL) i.e of standing length.
2-Fine lanugo hair, eye brows & head hair are also visible.
3-By the end of 5th month the body weight is about 500 grams( 1/ 7 of birth wt).
4-Fetal movement is easily recognized by the pregnant mother.
The weight of the fetus increases slowly during the first half of pregnancy,where as it increases rapidly during the 2nd half of pregnancy,specially in lthe last 2 months, where about 50% the full term baby weight is added.


At 6th to early 7th month
1-If borne ,it has no ability to survive because both the respiratory & nervous systems are not fully differentiated.
2-Has rounded appearance due to deposition of some fats under the skin& the
Skin is covered by Vernix Caseosa..
If born by the end of the 7th month(28 wks),then it is able to survive but with some difficulty &needs a special care in the Intensive Neonatal care Unit( Cubicles).

The Features at the end of the 9th month( at delivery or just before delivery)are:

1-The skull (Head) has the largest circumference of all parts of the body(12-13 cms)
,which forms about one fourth of the CHL( standing length is about 50 cms at birth).
2-The testes are now located in the scrotum ( i.e out side the body).
3-The weight at birth is a bout 3000-3400 grams( Average 3200 gms).
4- The CHL is about 50 cms & CRL is about 36 cms.

Some investigation are needed during fetal period as followings:

1-Ultrasonography to assess placental & fetal size,to diagnose Twins & Multiple pregnancy ,then to detect any congenital anomaly before birth of the baby.
2-Amniocentesis,usually done after the 14th week( aspirate sample of Amniotic fluid) for some biochemical analysis is cases of spina bifida(neural tube defect)& anencephaly where Alpha-Feto protein is increased.Also to detect some inborne Biochemical error.
Also to examine some fetal cells after culture & Giemsa stain.
3-Chorionic Vellous Sampling ( CVS),in order to do cell analysis with out culture for chromosomal & Biochemical analysis can be done from the 9th week of pregnancy.