Prenatal period

There are several systems for dividing the prenatal development of a person, e.g. 10 Jirasque stages (8 embryonic, 1 fetal, 1 prenatal), or 23 Carnegie stages. We evaluate the germ according to external characteristics. When evaluating the developmental stages of the embryo, it is important to distinguish between gestational and anatomical age.

Between the 3rd and 8th weeks of pregnancy, we refer to the developing individual as an embryo, after the 9th week as a fetus until the time of birthu (after that we already speak o newborn).

Periodization of intrauterine development
The periodization of intrauterine development can be based on several different characteristics:


 * according to time - but usually we don't know the exact moment of fertilization;
 * according to the size of the concept - the disadvantage here is individual differences, so it mainly applies statistically;
 * according to the number of somites - a fairly accurate method, but it can only be used between the 20th and 35th weeks (before the 20th week there are none, after the 35th week they merge);
 * by weight - there are also individual differences, besides embryo cannot be weighed based on ultrasound examination;
 * according to morphological characteristics - the most accurate method, can be compared with other animal species, e.g. Jirásk's periodization.

Prenatal development can be divided into three main periods. The first period is 'embryonicnal, during this period cells, tissues and organs are created, it lasts about 8 weeks. The next period is the fetal period. The last period is around birth and is the perinatal period.

Stages of development according to Jirásek

 * 1) Unicellular − begins fertilizationm, ootid, zygote, 1. day.
 * 2) Blastomeric - the oocyte begins to furrow (it divides, but its volume does not increase), a structure called the morula is formed, 2.−3. day.
 * 3) Blastoderms - a blastocyst is formed (fluid begins to enter the space between the cells until a uniform cavity is formed), 4. day.
 * 4) Double-layer target - epiblast, hypoblast, 2. week.
 * 5) Three-layer target with axial structures - 3 germ sheets are formed, 3. week.
 * 6) Tubular embryo - formation of somites occurs, closure of the neural tube, 4. week.
 * 7) C-shaped embryo - the embryo bends into lordosis, the formation of limbs, 5.−6. week.
 * 8) Late embryonic period - limbs (including fingers) are formed, at the end the eye slits close → end of embryonic period.
 * 9) Fetal period - end of the first trimester (after the end of the embryonic period), second and third trimester.
 * 10) Perinatal period.

Week 3
The main process of the third week of development is gastrulation, i.e. the invagination of the epiblast cells, which thus become the source of all three layers of the three-layered germ target. The formation of notochord (chorda dorsalis) occurs through the stage of the chordomesoderm process. We observe the formation of the tail tubercle, the first somites, the neural plate and the neural furrow.

Week 4
Creases occur:


 * Headfold:
 * arching of the brain into the amniotic cavity, then overhanging the heart;
 * septum transversum, base of the heart, pericardial cavity, oropharyngeal;
 * the membrane passes to the ventral side of the embryo;
 * between the region of the brain and the heart comes the anterior gut (separated by the oropharyngeal membrane);
 * there is a change in the shape of the intraembryonic coelom.
 * Caudal fold:
 * growth of spinal cord bases in length;
 * the rear intestine with a widened end - the cloaca is taken here;
 * allantois is partially incorporated.
 * Lateral folds:
 * rapid growth of spinal cord and somites;
 * the edges bend and wrap the edges of the germinal target ventrally – the embryo thus acquires a cylindrical shape;
 * midgut involved – connection limited to yolk sac volcano.

The anterior (25th day) and posterior (27th day) neuropores close, forebrain and somites emerge.

Other structures emerging during the 4th week:
 * pharyngeal arches;
 * limb buds;
 * the heart begins to pump blood;
 * the foundations of the liver, lungs, gall bladder, pancreas, urorectal septum are formed;
 * auditory sacs, lens placodes appear;
 * the oropharyngeal membrane breaks;
 * facial protrusions.

Week 5

 * Head development (brain grows), facial processes come into contact with the heart;
 * sinus cervicalis – the 2nd arch outgrows the 3rd and 4th;
 * limb buds – upper paddle-like, lower fin-like;
 * mesonephric bar.

Week 6

 * Palmar discs - digital rays;
 * ear bumps merge, meatus acusticus externus;
 * pigment to the retina – the eye can see;
 * the head continues to enlarge and overlaps the cardiac hump;
 * the torso and neck begin to straighten;
 * first spontaneous movements, umbilical hernia.

Week 7

 * Notches appear between digital beams, limbs rotate;
 * yolk stalk, umbilical herniation;
 * Ossification of upper extremity begins, face formed.

Week 8

 * Fingers of upper extremity are separated, Fingers of lower extremity not yet;
 * typically human appearance;
 * vascular plexus of the scalp;
 * the eyelids begin to fuse, the eye slits close, which is considered the end of the embryonic period.

Fetal period
The fetal period begins at the 9th week, when all body structures are already established, and lasts until birthu, i.e. until the 38th week. From the 25th week of pregnancy (perinatal stages), in the case of a possible premature birth, we no longer generally speak of a fetus, but of a premature child.

Related links

 * Gametogenesis
 * Fertilization
 * Egg scoring
 * First week of embryo development
 * Second week of embryo development
 * Third week of embryo development
 * Fourth to Eighth Embryo Development Week