Anencephaly is a defect of the development of the central nervous system, and its condition is incompatible with survival. Prenatal detection of anencephaly by ultrasound is accurate and reliable at 10-14 weeks scan, in some cases earlier, but diagnosis at a very early pregnancy is still unreliable because calcification of skull is complete by 10 weeks.

Ultrasound features of anencephaly are different in the first and second trimesters. In the first-trimester "mickey-mouse" bi-lobar face are viewed and typically represents this pathology, besides, other features can be present such as an increase of amniotic fluid echogenicity, irregular head morphology, and crown-rump length significantly reduced.

In the second trimester, the typical appearance of anencephaly is the "frog eye" sign, as shown in our ultrasound images due to absent cranial bone/brain and bulging eyes. In addition, polyhydramnios may be present due to a decrease in fetal swallowing. 

Start folic acid supplementation before conception and throughout the first trimester has shown a decreased risk of neural defects in almost 50% of the pregnancies.

Images: MD Jose Roberto Festugatto.