After receiving an ultrasound, especially after a 3D or 4D ultrasound, many mothers-to-be are often told that the lateral ventricles are widened, there are fluid dark areas, and there is fluid accumulation, and then the mothers-to-be begin to agonize over a hundred things: Does the baby have a brain abnormality? Could there be hydrocephalus? Will the baby be born “stupid”? If the doctor adds the phrase “regular review”, it is even more difficult to rest in peace. What exactly is the lateral ventricle? How much widening is considered to be? Does it necessarily mean that it is abnormal if it is widened? The ventricles are the normal structure of the brain. The brain is not completely solid, just like the door and window slits, there are some cavities in the brain. These cavities are called ventricles, and the lateral ventricle is one of them, which contains cerebrospinal fluid. Usually we say “water in the brain”, but in fact there is water in the brain, which is the cerebrospinal fluid. Cerebrospinal fluid plays an important role in nourishing and protecting the spinal cord, regulating intracranial pressure, and containing chemicals that act as a buffer. Cerebrospinal fluid flows through these brain chambers, creating a circulation. If for some reason the cerebrospinal fluid circulation pathway is blocked, ventricular dilatation or hydrocephalus can occur. Currently, ultrasound is an effective test to evaluate the presence or absence of fetal lateral ventricles dilatation. As gestational weeks increase, the internal diameter of the lateral ventricles decreases, and the closer to term, the less likely the lateral ventricles are to be visualized. However, the width of the lateral ventricle should not reach or exceed 10 mm at any gestational week. 10-15 mm is considered mild dilatation, and more than 15 mm is considered severe dilatation (some authors consider 10-12 mm as mild dilatation, 12-15 mm as moderate dilatation, and more than 15 mm as severe dilatation). In other words, if the ultrasound reports a unilateral ventricular width of less than 10 mm, whether it is 8 mm or 6 mm, it means that the baby’s lateral ventricular width is normal. Lateral ventricle measurement is a routine measurement in midtrimester ultrasound and not an indication that “my baby has hydrocephalus,” as some mothers-to-be may think. What can cause fetal ventricular dilatation? Strictly speaking, ventricular dilatation is not a diagnosis, but simply a sonographic description that encompasses a variety of causes and degrees of dilatation. Some ventricular dilatation is a temporary fetal manifestation or a normal variant, a temporary delay in cerebrospinal fluid drainage, or it may be a transient widening of the ventricles due to excessive secretion; some ventricular dilatation is the result of impaired cerebrospinal fluid reflux; and there are organic central nervous system malformations that also manifest as ventricular dilatation. When lateral ventricular dilatation is detected, first, we need to understand the degree of lateral ventricular dilatation and clarify whether there is a combination of other intracranial structural abnormalities and whether there are other malformations associated with it extracranially. If there is a combination of other intracranial or extracranial structural anomalies, the need to terminate the pregnancy can be chosen on a case-by-case basis. If there is only a mild dilatation of the lateral ventricles without other abnormalities, most fetuses will not develop hydrocephalus and many lateral ventricular widening can disappear on their own as the gestational weeks increase. The prognosis is good if there is no progression; if the ventricular dilatation is 12 mm to 15 mm, or if there is progression, fetal head MRI should be recommended, and if necessary, fetal amniotic fluid or cord blood karyotype analysis can also be performed to exclude chromosomal abnormalities, and fetal neurobehavioral development should be closely followed after birth. The fetal neurobehavioral development should be closely followed after birth. In cases of ventricular dilatation >15 mm, there are often intracranial structural abnormalities, and those with severe malformations may choose to terminate the pregnancy. Most of the fetuses with mild ventricular dilatation will not develop hydrocephalus, so the mother-to-be does not need to worry too much, but continue to have regular maternity checkups and leave any abnormalities to the doctor.