Periodization of intrauterine development

The periodization of intrauterine development can be based on several different characteristics: Prenatal development can be divided into three main periods. The first period is the embryonic during this period cells, tissues and organs are created, it lasts about 8 weeks. The next period is the fetal. The last period is around the birth and is the perinatal period.
 * according to time − but we usually do not know the exact moment of fertilisation;
 * 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 already merge);
 * by weight - there are also individual differences, in addition, the embryo cannot be weighed based on an ultrasound examination;
 * according to morphological characteristics - the most accurate method, can be compared with other animal species, e.g. Jirásk's periodization.

Stages of development according to Jirásek

 * 1) Unicellular − begins with fertilisation, ootid, zygote, day 1.
 * 2) Blastomeric − furrowing of the oocyte begins (it divides, but its volume does not increase), a structure called the morula is formed, 2.−3. day.
 * 3) Blastoderm - a blastocyst is formed (fluid begins to enter the space between the cells until a uniform cavity is formed), 4th day.
 * 4) Double-layer target − epiblast, hypoblast, are formed in the 2nd week.
 * 5) Three-layer target with axial structures - 3 germ sheets are formed, 3rd week.
 * 6) Tubular embryo - formation of somites, closure of the neural tube, 4th 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 the embryonic period.
 * 9) Fetal period - the end of the first trimester (after the end of the embryonic period), the second and third trimesters.
 * 10) Perinatal period.

Related articles

 * Prenatal period: The first week of embryo development • Second week of embryo development • Third week of embryo development • Fourth to eighth week of embryo development