Diagnosing deviations from normal intrauterine development requires a detailed knowledge of the normal developement. Here fetal pathology closely coresponds with embryology. The duration of normal gestation is 280 days (which equals 40 weeks) counted from the first day of the last menstruation. Intrauterine development is traditionally divided into the folowing stages:

• embryonal: 1 – 8th week of the fetal developement (equals 3 – 10th week of gestation (w.g.))
• fetal: from 9th week of the fetal development, i.e. from 11th week of gestation.

Furthermore, fetal period can be theoretically divided into a prevailable> (0 to 20 w.g.) and a viable period (over 20 w.g.).

It is necessary to emphasise that this classification is only formal and does not reflect the actual ability to survive after the premature delivery. This ability depends especially on the stage of development of the respiratory tract and lung tissue in particular. Infants born before 28th w.g. suffer of serious health complications caused by insufficient production of aleveolar surfactant by aleveolar type 2 pneumocytes.

Note: gestational age of the fetus is calculated from the first day of the last menstruation. However, the actual age of the fetus as approximately 2 weeks less (due to the usual date of conception) To make things clearer, the gestational age terminology is usualy used.