Which one of the following congenital malformation of the fetus can be diagnosed in first trimester by ultrasound ?
As discussed in chapter on fetal malformation, anencephaly can be diagnosed as early as 8-10 weeks/first trimester in experienced hards.
Microcephaly:
USG in Obs and Gynae Callen 4/e, p 298, 299

It is decreased head size.
In these patients both brain mass and total cell number are reduced.
There is disproportion in size between the skull and face.
It is difficult to recognize this anomaly by ultrasound – A head circumference below 2 SD from the mean is used as a diagnostic criteria in midtrimester scan.

Holoprosencephaly is the presence of a single centrally placed cerebral ventricle and may be associated with midline abnormalities of the face.
With the use of vaginal probe it can be recognised in the beginning of midtrimester.
Anencephaly
Inencephaly
Microcephaly
Holoprosencephaly