[Fetal posterior cranial fossa in the second and third trimester].

Objective: To examine the normal range of the width of posterior cranial fossa (WPCF) in the second and third trimester by ultrasonography, and to investigate its relationship with fetal congenital and chromosome abnormality.

Methods: WPCF of 2484 fetus (gestational age from 14 to 41 weeks) was measured by ultrasonograph routinely, and the infants were followed up.

Results: In 2848 fetus, 2772 were normal and 76 were abnormal. WPCF increased before 32 weeks, decreased after 33 weeks, the largest value of WPCF was 13.4 mm. The occurrence rate of WPCF> or =8 mm in normal fetus was 8.84%, and that in abnormal fetus was 17.46%. Most fetuses with chromosome abnormality had normal WPCF in the second trimester, but some fetuses with remarkable broadening in the late stage. Some abnormal fetuses (such as water head, Dandy-Walker's syndrome etc) showed significant extension of WPCF.

Conclusion: WPCF increases before 32 weeks, decreases after 33 weeks;and can be easily measured during 29 - 32 weeks. WPCF of some fetus with chromosome abnormality or with congenital abnormality is remarkably broadened in the late stage. The fetus of WPCF> or =10 mm should be followed up closely, and antenatal diagnosis should be done if WPCF is more than 14 mm.