Previable Fetal and Placental Development

Abstract

No formal system of staging exists for the fetal period. Development during the whole fetal period (9 weeks to term) primarily concerns rapid body growth and further differentiation of organs and tissues. There are no dramatic changes in the external appearance and, therefore, greater emphasis is placed on crown-rump (CR) and crown-heel (CH) measurements to establish fetal developmental age (see Appendix II, Table II-2). The fetal period can be subdivided into the previable fetal period (9 to 18 weeks of development) and the viable fetal period (19 to 38 weeks of development). Between the 9th and the 18th week, the rate of body growth is very rapid. The CR length of the fetus increases from 30 to 180 mm. The weight of the previable fetus varies and is unreliable as an indicator of developmental age (Golbus and Berry, 1977).