The patient was an 18-year-old woman, 2 gravida, nullipara. She had no remarkable medical or family history. She had a natural pregnancy and did not visit a hospital during the first trimester of pregnancy. The gestational age at the time of her initial hospital visit was estimated to be 15 weeks via measurement of the fetus on an ultrasound. Fetal biometry and estimated fetal weight were measured and evaluated according to the standardization committee of fetal measurement of the Japanese Society of Ultrasound in Medicine []. Head circumference (HC) was calculated from the diameter of biparietal diameter (BPD) and occipito-frontal diameter using the formula for an eclipse and evaluated according to the Hadlock et al. []. The BPD and femur length were 33 mm and 21 mm, respectively. At that time, there were no major anomalies, including the size and morphology of the fetal head. The estimated date of confinement was determined via ultrasound because of an unknown last menstrual period. At 23 weeks and 0 days of gestation, an ultrasound revealed a BPD of 40 mm (-5.3 standard deviations, SD), and the measured section of the BPD was poorly visualized. Additionally, observation of the fetal head using transvaginal ultrasound did not provide useful information about the cranial and cerebral defects. At 27 weeks and 5 days of gestation, an ultrasound revealed a BPD of 48 mm (-6.7 SD), however, there was difficulty visualizing the skull above the orbit and the overall growth, except for the head, was within the normal size range. At 29 weeks and 0 days of gestation, an ultrasound revealed a BPD of 43.6 mm (-8.9 SD) and a HC of 15.19 mm (-12.4 SD). At 31 weeks and 6 days of gestation, we explained to the patient and the family that there was a possibility of microencephaly or anencephaly, and that anencephaly would result in a poor prognosis. We proposed magnetic resonance imaging (MRI) to the patient for a definitive diagnosis of the fetal head anomaly. However, the patient and the family refused further examination. At 34 weeks and 4 days of gestation, the patient was admitted for induction of labor due to the patient preferences. Elevation of the fetal head due to cervical dilatation by insertion of a metreurynter permitted confirmation of the structure of the fetal skull. MRI and 3-dimensional computed tomography (3D-CT) were performed to confirm the absence of anencephaly. 3D-CT revealed the presence of a fetal skull, a prominent occipital bone, and a sloping forehead. MRI showed a cerebral sickle, cerebellar tent, cerebral defects, a hypoplastic cerebellum, and a normal brain stem. The presence of the skull, cerebellum, and brainstem allowed us to diagnose the fetus with microhydranencephaly, rather than anencephaly. This diagnosis indicated that the fetus may be able to survive after delivery. Thus, the induction of labor was stopped, and the patient and the family were counseled for treatment. Vaginal delivery was scheduled with an exception for a cesarean section in the case of maternal indication. For the infant, noninvasive resuscitation treatment, except for tracheal intubation and chest compression, was scheduled for after delivery. Because the patient had cancelled many scheduled visits, she was hospitalized until delivery. At 36 weeks and 6 days of gestation, she had a premature rupture of the membranes, and delivered vaginally the following day without shoulder dystocia due to the small fetal head. The baby was a male weighing 2342 g, with a HC of 24 cm (-5.4 SD according to the Japanese neonatal data []) and Apgar scores of 1, 5, and 8 after 1, 5, and 10 min, respectively. Umbilical artery blood pH, pCO2, pO2, and base excess were 7.34, 42.7 mm Hg, 22.5 mm Hg, and − 3.5 mmol/L, respectively. After birth, the baby had spontaneous respiration. Both eyes and the nose had a normal appearance, but the head was flat above the forehead, with a suspected partial head defect with a sloping forehead. Skin defects were found in the parietal region, and encephalocele was suspected. The skin lesion was taped, and healing was confirmed after 12 days, with no signs of infection or leakage of cerebrospinal fluid. Although the sucking reflex was observed, swallowing movements appeared difficult. On day 11 after birth, a gastric tube was inserted, and tube feeding was started. On day 32 after birth, an MRI revealed the absence of cerebral tissue; instead, a membranous structure was present and hydrocephalus was evident, which was consistent with microhydranencephaly. Screenings for rubella virus, herpesvirus, cytomegalovirus, and toxoplasma infections causing hydrocephalus were all negative. Chromosomal analysis with G-banding yielded 46, XY, with and inv (9) (p12 q13) harboring less clinically significant abnormalities. The baby received desmopressin acetate due to central diabetes insipidus 6 months after birth.