During the first few weeks of life, the Carnegie's stage provides the best system for assessment of the embryonic age. In the third and early parts of the fourth week the embryo is measured along its length. Later, the sitting height or crown to rump length (CRL) is used for measuring the embryo. In 8 week old embryos, one, can also measure the crown to heel length (CHL) for estimating the age. Additional measurements that are helpful for estimating the age of the embryo are the dimensions of the head and trunk. Immediately before and after birth, the size of the head is used for assessment of the growth and for determining the growth retardation of the embryo.