The period from the 3rd to 8th week of the development is known as the embryonic period. The embryonic period is very important because this is the time when all internal and external structures develop in the embryo. In the period from the 4th to the 8th week, each of the three germ layers gives rise to different tissues and organs. As the result of the organogenesis, the shape of an embryo greatly changes. During this critical period, the exposure of an embryo to certain agents may cause major congenital malformations. The period from the beginning of the 3rd month to the birth is known as the fetal period. The main characteristic of the fetus in this period is the rapid growth of the body and the maturation of the tissues. The fetal growth can be monitored with ultrasound.

During the 4th week of the development, the age of the embryo is expressed in the number of somites.
The length of the embryo between the 4th and the 8th developmental weeks is indicated as the crown-rump length (CRL) and is expressed in millimeters. During fetal development (9th - 40th week) the length of the conceptus is indicated as CRL or as the crown-heel length (CHL) expressed in centimeters. External appearance, weight or biparietal diameter are also used as a measures during the gestation.
Growth in length is most intensive during the 3rd to the 5th month, while the increase in weight is most evident during the last two months of gestation. During the fetal life, head growth rate slows down. During the 5th month, the mother can recognize fetal movements. The birth occurs 266 days or 38 weeks after the fertilization.

External appearance - 6th week



External appearance - 11th week