Development of the Fetus
The period from the beginning of the ninth week to birth is known as the fetal period. It is characterized by maturation of tissues and organs and rapid growth of the body The length of the fetus is usually indicated as the crown-rump length (CRL) (sitting height) or as the crown-heel length (CHL), the measurement from the vertex of the skull to the heel (standing height). These measurements, expressed in centimeters, are correlated with the age of the fetus in weeks or months (Table 6.1; p. 118). Growth in length is particularly striking during the third, fourth, and fifth months, while an increase in weight is most striking during the last 2 months of gestation. In general, the length of pregnancy is considered to be 280 days, or 40 weeks after the onset of the last normal menstrual period (LNMP) or more accurately, 266 days or 38 weeks after fertilization. For the purposes of the following discussion, age is calculated from the time of fertilization and is expressed in weeks or calendar months.
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