Accurate diagnosis of anencephaly in ultrasound is seen at week:
Friend, there are two aspects which should be considered while answering such a question.

Best time for diagnosis
Earliest time of detection.

In case of anencephaly:
“The diagnosis may be suspected in the first trimester during examination with endovaginal probe but it is usually not certain until after 14 weeks.”
Fernando Arias 3/e, p 64
“Anencephaly can theoretically be diagnosed as early as 8 weeks; however it can be missed in the first trimester. In the second trimester, if an adequate ultrasound examination can be performed, anencephaly is diagnosed with virtually 100% accuracy.”
Williams Obs. 21/e, p 1120
“It can be diagnosed in the late first trimester and with adequate visualization, virtually all cases may be diagnosed in the second trimester”.
Williams Obs. 23/e, p 354
Anencephaly -
“Although fetal head can be positively identified by vaginal sonography as early as the 7th week of gestation, the diagnosis may be difficult in the first trimester. The diagnosis is easy in the midtrimester.”
Callen, Ultrasound in Obs. & Gynae 4/e, p 284
6 weeks of gestation
8 weeks of gestation
10 weeks of gestation
14 weeks of gestation