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.