Icon module 8

Learning aims

At the end of this module you should know:

  • The differentiations of the germinal layers during the fourth week of development that lead to an individualization of the embryo.
  • The key concepts of the embryonic period that describe the first stages of organogenesis.
  • The various types of congenital abnormalities and be able to cite a few characteristic examples.

What you should already know

  • Preimplantation
  • Implantation
  • Embryonic disk

Delving deeper

  • Which relationships can exist between the sequence of organ development and the risks for congenital abnormalities?
  • What are the factors that regulate the proliferation and differentiation of the cells and thus lead to the formation of the organs?
  • With which methods can the growth of each embryo be examined in order to ascertain its development (whether it is normal or pathologic)?


The embryonic period ranges from the fertilization up to the 8th week of development.

The fetal period comprises that from the 9th week (3rd month) up to delivery and thus lasts much longer.

The embryonic period is divided into a preembryonic period (from the 1rst to the 3rd week) (stages 1-8), which includes fertilization, the implantation and the formation of the bi- and tri-laminar embryo, and into the true embryonic period (3rd to the 8th week) (stages 9-23).

During the embryonic period specific tissues and organs arise from each of the three germinal layers. It is the time period of organogenesis in which the embryo takes on its human form and increases its weight from 1/1'000th of a mg to 2-3 g (an increase of 2-3'000'000!).

Fig. 1 - Temporal divisions during a pregnancy

Embryonic period
Fetal period

Fig. 1

LMP = date of the last menstrual period.
The embryonic period (A) lasts 8 weeks.
The fetal period (B) goes from the 9th week to the delivery.