The 'Mickey Mouse' sign and the diagnosis of anencephaly in early pregnancy.

OBJECTIVES

To assess the sonographic screening for anencephaly in the first trimester in a low-risk obstetric population.

METHODS

Since 1994, 5388 women attended our clinic for a first-trimester scan (11-14 weeks of gestation) and screening for structural and chromosomal abnormalities. The patients underwent transabdominal scanning, and transvaginal scanning if necessary.

RESULTS

The ultrasonographic appearances of anencephaly in the first trimester are different from the familiar second-trimester signs. The cerebral hemispheres are present and exposed to the surrounding amniotic fluid. The ultrasound appearances in the coronal section of the head are best described as 'Mickey Mouse face'. There were six cases of anencephaly (incidence 1.1:1000). All cases were diagnosed in the first trimester and five demonstrated this sign. There were no false-positive diagnoses. The crown-rump length was significantly reduced in all affected fetuses.

CONCLUSION

First-trimester ultrasonographic diagnosis of anencephaly is accurate, but sonographers should be familiar with the ultrasound appearances that are different from those in the second trimester.